libnvme: annotate public API symbols with __public

Add compiler_attributes.h defining two GCC visibility macros:

  __public  (__attribute__((visibility("default"))))
  __weak    (__attribute__((weak)))

Annotate all functions exported via libnvme.ld, libnvmf.ld, and
accessors.ld with __public. This prepares libnvme for builds with
-fvisibility=hidden, ensuring internal symbols are hidden by default
and only explicitly annotated functions form the public ABI.

Each affected .c file now includes compiler_attributes.h and marks
the relevant function definitions with __public. The accessor generator
has also been updated to emit __public, so future regenerations remain
consistent.

As a small related cleanup, __weak was introduced in
compiler_attributes.h and applied where needed. While not strictly part
of the main change, it naturally fits alongside the new attribute
definitions and keeps related compiler annotations consolidated.

Signed-off-by: Martin Belanger <martin.belanger@dell.com>

Author: Martin Belanger

libnvme/src/meson.build

@@ -96,6 +96,7 @@ libnvme = library(
     'nvme', # produces libnvme.so
     sources,
     version: libnvme_so_version,
+    c_args: ['-fvisibility=hidden'],
     link_args: link_args,
     dependencies: deps,
     install: true,