Currently, there isn't a reliable way to build

Go test binaries with libfuzzer instrumentation [1].
An attempt such as
`bazel run --@io_bazel_rules_go//go/config:gc_goopts=-d=libfuzzer ...`
fails to compile owing to unresolved symbols, e.g.,
runtime.libfuzzerTraceConstCmp8 [2].

This patch adds `libfuzzer_shim.go` (identical to `trace.go` [2])
to the `bzltestutil` package, which ensures the shim is linked
with a Go test binary. Furthermore, we exclude `-d=libfuzzer`
when compiling any of the _external_ dependencies.

[1] bazel-contrib#3088 (comment)
[2] golang/go@74f49f3

go/tools/builders/compilepkg.go

@@ -27,6 +27,7 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"slices"
 	"sort"
 	"strings"
 )
@@ -343,6 +344,13 @@ func compileArchive(
 			cgoSrcs[i-len(goSrcs)] = coverSrc
 		}
 	}
+	if strings.HasPrefix(packagePath, "external") && slices.Contains(gcFlags, "-d=libfuzzer") {
+		// Remove -d=libfuzzer from gcFlags when compiling external packages. We don't really want to instrument them,
+		// and they won't compile without libfuzzer_shim.go.
+		gcFlags = slices.DeleteFunc(gcFlags, func(s string) bool {
+			return s == "-d=libfuzzer"
+		})
+	}
 
 	// If we have cgo, generate separate C and go files, and compile the
 	// C files.

go/tools/bzltestutil/libfuzzer_shim.go

@@ -0,0 +1,41 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// N.B. This source is lifted verbatim from trace.go, added in
+// https://github.com/golang/go/commit/74f49f3366826f95a464cc15838a0668c92e3357
+//
+// It's essentially a shim to allow linking a Go test binary without libfuzzer.
+// We chose the 'bzltestutil' package because it's a bazel dependency for all Go test binaries.
+
+//go:build !libfuzzer
+
+package bzltestutil
+
+import _ "unsafe" // for go:linkname
+
+//go:linkname libfuzzerTraceCmp1 runtime.libfuzzerTraceCmp1
+//go:linkname libfuzzerTraceCmp2 runtime.libfuzzerTraceCmp2
+//go:linkname libfuzzerTraceCmp4 runtime.libfuzzerTraceCmp4
+//go:linkname libfuzzerTraceCmp8 runtime.libfuzzerTraceCmp8
+
+//go:linkname libfuzzerTraceConstCmp1 runtime.libfuzzerTraceConstCmp1
+//go:linkname libfuzzerTraceConstCmp2 runtime.libfuzzerTraceConstCmp2
+//go:linkname libfuzzerTraceConstCmp4 runtime.libfuzzerTraceConstCmp4
+//go:linkname libfuzzerTraceConstCmp8 runtime.libfuzzerTraceConstCmp8
+
+//go:linkname libfuzzerHookStrCmp runtime.libfuzzerHookStrCmp
+//go:linkname libfuzzerHookEqualFold runtime.libfuzzerHookEqualFold
+
+func libfuzzerTraceCmp1(arg0, arg1 uint8, fakePC int)  {}
+func libfuzzerTraceCmp2(arg0, arg1 uint16, fakePC int) {}
+func libfuzzerTraceCmp4(arg0, arg1 uint32, fakePC int) {}
+func libfuzzerTraceCmp8(arg0, arg1 uint64, fakePC int) {}
+
+func libfuzzerTraceConstCmp1(arg0, arg1 uint8, fakePC int)  {}
+func libfuzzerTraceConstCmp2(arg0, arg1 uint16, fakePC int) {}
+func libfuzzerTraceConstCmp4(arg0, arg1 uint32, fakePC int) {}
+func libfuzzerTraceConstCmp8(arg0, arg1 uint64, fakePC int) {}
+
+func libfuzzerHookStrCmp(arg0, arg1 string, fakePC int)    {}
+func libfuzzerHookEqualFold(arg0, arg1 string, fakePC int) {}