[GOLD] Support --icf=safe with -pie for x86_64

[Cary Coutant]

>         * x86_64.cc (Target_x86_64::do_can_check_for_function_pointers):
>         Return true even when building pie binaries.
>         (Target_x86_64::possible_function_pointer_reloc): Check opcode
>         for R_X86_64_PC32 relocations.
>         (Target_x86_64::local_reloc_may_be_function_pointer): Pass
>         extra arguments to local_reloc_may_be_function_pointer.
>         (Target_x86_64::global_reloc_may_be_function_pointer): Likewise.
>         * gc.h (gc_process_relocs): Add check for STT_FUNC.
>         * testsuite/Makefile.am (icf_safe_pie_test): New test case.
>         * testsuite/Makefile.in: Regenerate.
>         * testsuite/icf_safe_pie_test.sh: New shell script.

This is OK. I've committed it on your behalf.

Thanks!

-cary