Add SHT_X86_64_UNWIND to elf.h (bug 20079).

As requested in bug 20079, this patch adds SHT_X86_64_UNWIND (a
standard value from the x86_64 ABI) to elf.h.

Tested for x86_64.

	[BZ #20079]
	* elf/elf.h (SHT_X86_64_UNWIND): New macro.