ld: Provide a way to make C identifier name sections GCable under __start_/__stop_ references

[Fangrui Song]

Hi! I have noticed an interesting problem with GC on C identifier name sections and just created
https://sourceware.org/bugzilla/show_bug.cgi?id=27451 .
(
More details are in a write-up:
https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order
You can skip the part before "Metadata sections"
)

I'll not repeat the message on the feature request. This thread is to
attract folks who don't subscribe to bug-binutils@gnu.org
The main request is that we may need a linker option (-z start-stop-gc?) to drop the original rule,
changing

     __start_/__stop_ references from a live section retain all the associated C identifier name sections.

to

     __start_/__stop_ references from a live section do not retain the associated C identifier name sections.

     (There is a clang -fsanitize-coverage feature which demands the new behavior.

     clang PGO struggled with the original rule as well. It can benefit from the new behavior, or a SHF_GROUP special
     case once GNU ld implements it.)