Define SIG_HOLD for XPG4 (bug 21538)

[Zack Weinberg]

On 06/01/2017 01:45 PM, Joseph Myers wrote:
> 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.

OK.