gold: x86-64: Fix TLSDESC -> LE relaxation

gold: x86-64: Fix TLSDESC -> LE relaxation

X86-64 TLSDESC sequences can be:

4c 8d 0d 00 00 00 00	leaq	foo@TLSDESC(%rip), %r9
4c 89 c8		movq	%r9, %rax
ff 10			call	*foo@TLSCALL(%rax)

TLSDESC -> LE relaxation can turn them into:

49 c7 c1 fc ff ff ff 	mov    $0xfffffffffffffffc,%r9
4c 89 c8             	mov    %r9,%rax
66 90                	xchg   %ax,%ax

We need to check and update the REX byte in this case.

	PR gold/25473
	* x86_64.cc (Target_x86_64<size>::Relocate::tls_desc_gd_to_ie):
	Properly check r8 - r15 in "lea foo@TLSDESC(%rip), %reg".
	(Target_x86_64<size>::Relocate::tls_desc_gd_to_le): Properly
	relax r8 - r15 in "lea foo@TLSDESC(%rip), %reg".
	* testsuite/Makefile.am (check_SCRIPTS): Add x86_64_gd_to_le.sh.
	(check_DATA): Add x86_64_gd_to_le.stdout.
	(MOSTLYCLEANFILES): Add x86_64_gd_to_le.
	(x86_64_gd_to_le.o): New target.
	(x86_64_gd_to_le): Likewise.
	(x86_64_gd_to_le.stdout): Likewise.
	* testsuite/Makefile.in: Regenerated.
	* testsuite/x86_64_gd_to_le.s: New file.
	* testsuite/x86_64_gd_to_le.sh: Likewise.