unix: add PthreadSigmask for Linux

Add a syscall wrapper for SYS_RT_SIGPROCMASK and export it as
PthreadSigmask. The latter is defined by POSIX and can therefore
be implemented by Darwin, etc. later on.

Follow the approach used by Signalfd of passing _C__NSIG/8 as
sigsetsize. This avoids exporting _C__NSIG and allows the syscall
to work with the current definition of Sigset_t, which doesn't
match the kernel definition of Sigset_t.

Updates golang/go#55349

unix/syscall_linux.go

@@ -2368,6 +2368,16 @@ func Setitimer(which ItimerWhich, it Itimerval) (Itimerval, error) {
 	return prev, nil
 }
 
+//sysnb	rtSigprocmask(how int, set *Sigset_t, oldset *Sigset_t, sigsetsize uintptr) (err error) = SYS_RT_SIGPROCMASK
+
+func PthreadSigmask(how int, set, oldset *Sigset_t) error {
+	if oldset != nil {
+		// Explicitly clear in case Sigset_t is larger than _C__NSIG.
+		*oldset = Sigset_t{}
+	}
+	return rtSigprocmask(how, set, oldset, _C__NSIG/8)
+}
+
 /*
  * Unimplemented
  */
@@ -2426,7 +2436,6 @@ func Setitimer(which ItimerWhich, it Itimerval) (Itimerval, error) {
 // RestartSyscall
 // RtSigaction
 // RtSigpending
-// RtSigprocmask
 // RtSigqueueinfo
 // RtSigreturn
 // RtSigsuspend