X32: Add REX prefix to encode R_X86_64_GOTTPOFF

Structions with R_X86_64_GOTTPOFF relocation must be encoded with REX
prefix even if it isn't required by destination register.  Otherwise
linker can't safely perform IE -> LE optimization.

bfd/

	PR ld/17482
	* elf64-x86-64.c (elf_x86_64_relocate_section): Update comments
	for IE->LE transition.

gas/

	PR ld/17482
	* config/tc-i386.c (output_insn): Add a dummy REX_OPCODE prefix
	for structions with R_X86_64_GOTTPOFF relocation for x32 if needed.

gas/testsuite/

	PR ld/17482
	* gas/i386/ilp32/x32-tls.d: New file.
	* gas/i386/ilp32/x32-tls.s: Likewise.

ld/testsuite/

	PR ld/17482
	* ld-x86-64/tlsie4.dd: Updated.
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