Define SIG_HOLD for XPG4 (bug 21538).

Define SIG_HOLD for XPG4 (bug 21538).

Various bits/signum.h headers define SIG_HOLD if __USE_UNIX98.  That
should be __USE_XOPEN, as this macro is in XPG4.  This patch fixes the
conditionals accordingly.  Because of other header bugs, this does not
allow any XFAILs to be removed (however, the XPG4/signal.h/conform
XFAIL only depends on a few such straightforward header bugs, not on
the more complicated to fix ucontext_t issues, as ucontext_t isn't
included in signal.h in XPG4).

Tested for x86_64.

	[BZ #21538]
	* bits/signum.h (SIG_HOLD): Define if [__USE_XOPEN], not
	[__USE_UNIX98].
	* sysdeps/unix/bsd/bits/signum.h (SIG_HOLD): Likewise.
	* sysdeps/unix/sysv/linux/alpha/bits/signum.h (SIG_HOLD):
	Likewise.
	* sysdeps/unix/sysv/linux/bits/signum.h (SIG_HOLD): Likewise.
	* sysdeps/unix/sysv/linux/hppa/bits/signum.h (SIG_HOLD): Likewise.
	* sysdeps/unix/sysv/linux/mips/bits/signum.h (SIG_HOLD): Likewise.
	* sysdeps/unix/sysv/linux/sparc/bits/signum.h (SIG_HOLD):
	Likewise.