gc: alternative clang compatible abort.

Tested on clang 2.9/amd64

R=rsc, jeff, r
CC=golang-dev
https://golang.org/cl/4517143

src/cmd/gc/subr.c

@@ -105,7 +105,7 @@ hcrash(void)
 		flusherrors();
 		if(outfile)
 			unlink(outfile);
-		abort();
+		*(volatile int*)0 = 0;
 	}
 }