Fix another x86 sys/ucontext.h namespace issue (bug 21457).

This patch fixes a namespace issue for one more field in the x86
sys/ucontext.h that I missed in my previous changes.

Tested for x86_64.

	[BZ #21457]
	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h [__x86_64__] (struct
	_libc_xmmreg): Use __ctx in defining field.