gc: enable building under clang/2.9

To build under clang, pass the path to clang in CC when
calling ./make.bash

CC=/opt/llvm/llvm-2.9/bin/clang ./make.bash

Credit goes to jmhodges for suggestions.

R=jeff, r, ality
CC=golang-dev
https://golang.org/cl/4527098

src/cmd/gc/subr.c

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