Improved support for --icf=safe when used with -pie.

Improved support for --icf=safe when used with -pie.

gold/
	* 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.