Try to make sure struct _mcontext is 16-byte aligned

Try to make sure struct _mcontext is 16-byte aligned

On x86_64, RtlCaptureContext() uses fxsave to save FPU/MMX/SSE state.

fxsave requires that the destination address is 16-byte aligned, or it will
fault.

CONTEXT is already annotated __attribute__ ((aligned (16))), do the same with
struct _mcontext.

Rearrange ucontext_t so that it's struct _mcontext element is also correctly
aligned.

	* include/cygwin/signal.h (struct __mcontext): 16-byte align.
	* include/sys/ucontext.h (ucontext_t): Ditto.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>