runtime: add a missing hex conversion

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/13879
Reviewed-by: Ian Lance Taylor <iant@golang.org>