cc: Add cross_cpu arg to nixpkgs_cc_toolchain_config

Currently, we hardcode the CPU architecture of the toolchain to the host toolchain.
However, this breaks cross compilation as we will probably want to set the CPU
of the toolchain to the target architecture. The `cross_cpu` flag can be used to
specify the target architecture. If the `cross_cpu` flag is not set, we will auto detect the
host CPU architecture and use that for the toolchain.

Is part of an effort to address tweag#347

toolchains/cc/cc.bzl

@@ -31,9 +31,9 @@ load("@bazel_skylib//lib:sets.bzl", "sets")
 load("@rules_nixpkgs_core//:nixpkgs.bzl", "nixpkgs_package")
 load(
     "@rules_nixpkgs_core//:util.bzl",
-    "is_bazel_version_at_least",
     "ensure_constraints",
     "execute_or_fail",
+    "is_bazel_version_at_least",
 )
 
 def _parse_cc_toolchain_info(content, filename):
@@ -121,8 +121,9 @@ def _parse_cc_toolchain_info(content, filename):
     )
 
 def _nixpkgs_cc_toolchain_config_impl(repository_ctx):
-    cpu_value = get_cpu_value(repository_ctx)
-    darwin = cpu_value == "darwin" or cpu_value == "darwin_arm64"
+    host_cpu = get_cpu_value(repository_ctx)
+    cross_cpu = repository_ctx.attr.cross_cpu or host_cpu
+    darwin = host_cpu == "darwin" or host_cpu == "darwin_arm64"
 
     cc_toolchain_info_file = repository_ctx.path(repository_ctx.attr.cc_toolchain_info)
     if not cc_toolchain_info_file.exists and not repository_ctx.attr.fail_not_supported:
@@ -147,7 +148,7 @@ def _nixpkgs_cc_toolchain_config_impl(repository_ctx):
 
     # A module map is required for clang starting from Bazel version 3.3.0.
     # https://github.com/bazelbuild/bazel/commit/8b9f74649512ee17ac52815468bf3d7e5e71c9fa
-    bazel_version_match, bazel_from_source  = is_bazel_version_at_least("3.3.0")
+    bazel_version_match, bazel_from_source = is_bazel_version_at_least("3.3.0")
     needs_module_map = info.is_clang and (bazel_version_match or bazel_from_source)
     if needs_module_map:
         generate_system_module_map = [
@@ -186,7 +187,7 @@ def _nixpkgs_cc_toolchain_config_impl(repository_ctx):
         repository_ctx.path(repository_ctx.attr._build),
         {
             "%{cc_toolchain_identifier}": "local",
-            "%{name}": cpu_value,
+            "%{name}": cross_cpu,
             "%{modulemap}": ("\":module.modulemap\"" if needs_module_map else "None"),
             "%{supports_param_files}": "0" if darwin else "1",
             "%{cc_compiler_deps}": get_starlark_list(
@@ -199,7 +200,7 @@ def _nixpkgs_cc_toolchain_config_impl(repository_ctx):
             "%{abi_libc_version}": "local",
             "%{host_system_name}": "local",
             "%{target_libc}": "macosx" if darwin else "local",
-            "%{target_cpu}": cpu_value,
+            "%{target_cpu}": cross_cpu,
             "%{target_system_name}": "local",
             "%{tool_paths}": ",\n        ".join(
                 ['"%s": "%s"' % (k, v) for (k, v) in info.tool_paths.items()],
@@ -224,6 +225,7 @@ _nixpkgs_cc_toolchain_config = repository_rule(
     attrs = {
         "cc_toolchain_info": attr.label(),
         "fail_not_supported": attr.bool(),
+        "cross_cpu": attr.string(),
         "_unix_cc_toolchain_config": attr.label(
             default = Label("@bazel_tools//tools/cpp:unix_cc_toolchain_config.bzl"),
         ),
@@ -246,7 +248,7 @@ _nixpkgs_cc_toolchain_config = repository_rule(
 )
 
 def _nixpkgs_cc_toolchain_impl(repository_ctx):
-    cpu = get_cpu_value(repository_ctx)
+    cpu = repository_ctx.attr.cross_cpu or get_cpu_value(repository_ctx)
     exec_constraints, target_constraints = ensure_constraints(repository_ctx)
 
     repository_ctx.file(
@@ -287,6 +289,7 @@ _nixpkgs_cc_toolchain = repository_rule(
         "cc_toolchain_config": attr.string(),
         "exec_constraints": attr.string_list(),
         "target_constraints": attr.string_list(),
+        "cross_cpu": attr.string(),
     },
 )
 
@@ -304,7 +307,8 @@ def nixpkgs_cc_configure(
         exec_constraints = None,
         target_constraints = None,
         register = True,
-        cc_lang = "c++"):
+        cc_lang = "c++",
+        cross_cpu = ""):
     """Use a CC toolchain from Nixpkgs. No-op if not a nix-based platform.
 
     By default, Bazel auto-configures a CC toolchain from commands (e.g.
@@ -375,8 +379,8 @@ def nixpkgs_cc_configure(
       target_constraints: Constraints for the target platform.
       register: bool, enabled by default, Whether to register (with `register_toolchains`) the generated toolchain and install it as the default cc_toolchain.
       cc_lang: string, `"c++"` by default. Used to populate `CXX_FLAG` so the compiler is called in C++ mode. Can be set to `"none"` together with appropriate `copts` in the `cc_library` call: see above.
+      cross_cpu: string, `""` by default. Used if you want to add a cross compilation C/C++ toolchain. Set this to the CPU architecture for the target CPU. For example x86_64, would be k8.
     """
-
     nixopts = list(nixopts)
     nix_file_deps = list(nix_file_deps)
 
@@ -447,6 +451,7 @@ def nixpkgs_cc_configure(
         name = "{}".format(name),
         cc_toolchain_info = "@{}_info//:CC_TOOLCHAIN_INFO".format(name),
         fail_not_supported = fail_not_supported,
+        cross_cpu = cross_cpu,
     )
 
     # Generate the `cc_toolchain` workspace.
@@ -457,6 +462,7 @@ def nixpkgs_cc_configure(
         cc_toolchain_config = name,
         exec_constraints = exec_constraints,
         target_constraints = target_constraints,
+        cross_cpu = cross_cpu,
     )
 
     if register: