Fix signal stack namespace (bug 21584).

In POSIX.1:2008, various signal stack symbols - sigaltstack, SS_*,
SIGSTKSZ, MINSIGSTKSZ - are all XSI-shaded.  glibc wrongly makes them
visible for non-XSI POSIX.  This patch fixes the conditionals, leaving
the symbols available for __USE_MISC as it seems likely some of them
are widely used and should be visible by default.  (Note that stack_t
is *not* XSI-shaded and so the conditionals on that are correctly
unchanged.)

Tested for x86_64.

	[BZ #21584]
	* signal/signal.h: Make includes of <bits/sigstack.h> and
	<bits/ss_flags.h> conditional on [__USE_XOPEN_EXTENDED ||
	__USE_MISC].
	(sigaltstack): Make declaration conditional on
	[__USE_XOPEN_EXTENDED || __USE_MISC].