runtime: initialize variable to avoid compiler warning

    
    Reviewed-on: https://go-review.googlesource.com/13095

From-SVN: r226543

gcc/go/gofrontend/MERGE

-a850225433a66a58613c22185c3b09626f5545eb
+bdd98c601f2c8dbd0bf821548ba09c038f7645c4
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/mprof.goc

@@ -403,7 +403,7 @@ func ThreadCreateProfile(p Slice) (n int, ok bool) {
 
 func Stack(b Slice, all bool) (n int) {
 	byte *pc;
-	bool enablegc;
+	bool enablegc = false;
 	
 	pc = (byte*)(uintptr)runtime_getcallerpc(&b);