Replace __int128 with __int128_t in bits/link.h

__int128 was added in GCC 4.6 and __int128_t was added before x86-64
was supported.  This patch replaces __int128 with __int128_t so that
the installed bits/link.h can be used with older GCC.

	* sysdeps/x86/bits/link.h (La_x86_64_regs): Replace __int128
	with __int128_t.
	(La_x86_64_retval): Likewise.