Gold: Enable safe ICF for shared object on x86-64

Gold: Enable safe ICF for shared object on x86-64

With

commit 4aebb6312eb5dcd12f2f8420028547584b708907
Author: Rahul Chaudhry <rahulchaudhry@google.com>
Date:   Wed Feb 15 00:37:10 2017 -0800

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

we now check opcode with R_X86_64_PC32 relocation, which tell branches
from other instructions.  We can enable safe ICF for shared object on
x86-64.  Also, global symbols with non-default visibility should be
folded like local symbols.

	PR gold/21452
	* x86_64.cc (Scan::local_reloc_may_be_function_pointer): Remove
	check for shared library.
	(Scan::global_reloc_may_be_function_pointer): Remove check for
	shared library and symbol visibility.
	* testsuite/icf_safe_so_test.cc (bar_static): New function.
	(main): Take function address of bar_static and use it.
	* testsuite/icf_safe_so_test.sh (arch_specific_safe_fold): Also
	check fold on x86-64.  Check bar_static isn't folded.