runtime: make gsignal stack at least SIGSTKSZ bytes

The default stack size for the gsignal goroutine, 32K, is not enough on
    ia64.  Make sure that the stack size is at least SIGSTKSZ.
    
    Reviewed-on: https://go-review.googlesource.com/28224

From-SVN: r239894

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-394486a1cec9bbb81216311ed153179d9fe1c2c5
+c8cf90f2daf62428ca6aa0b5674572cd99f25fe3
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/runtime.c

@@ -272,7 +272,14 @@ runtime_tickspersecond(void)
 void
 runtime_mpreinit(M *mp)
 {
-	mp->gsignal = runtime_malg(32*1024, (byte**)&mp->gsignalstack, &mp->gsignalstacksize);	// OS X wants >=8K, Linux >=2K
+	int32 stacksize = 32 * 1024;	// OS X wants >=8K, Linux >=2K
+
+#ifdef SIGSTKSZ
+	if(stacksize < SIGSTKSZ)
+		stacksize = SIGSTKSZ;
+#endif
+
+	mp->gsignal = runtime_malg(stacksize, (byte**)&mp->gsignalstack, &mp->gsignalstacksize);
 	mp->gsignal->m = mp;
 }