runtime: add a missing hex conversion

gobuf.g is a guintptr, so without hex(), it will be printed as
a decimal, which is not very helpful and inconsistent with how
other pointers are printed.

Change-Id: I7c0432e9709e90a5c3b3e22ce799551a6242d017
Reviewed-on: https://go-review.googlesource.com/13879Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/stack1.go

@@ -669,7 +669,7 @@ func newstack() {
 		throw("stack growth after fork")
 	}
 	if thisg.m.morebuf.g.ptr() != thisg.m.curg {
-		print("runtime: newstack called from g=", thisg.m.morebuf.g, "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")
+		print("runtime: newstack called from g=", hex(thisg.m.morebuf.g), "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")
 		morebuf := thisg.m.morebuf
 		traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
 		throw("runtime: wrong goroutine in newstack")