Align stack to 16 bytes when calling __setcontext

Don't use pop to restore %rdi so that stack is aligned to 16 bytes
when calling __setcontext.

	[BZ #18661]
	* sysdeps/unix/sysv/linux/x86_64/__start_context.S
	(__start_context): Don't use pop to restore %rdi so that stack
	is aligned to 16 bytes when calling __setcontext.