Being able to printer pointers to strings means one will able to output
the result of things like the flag library and other components that use
string pointers.

While here, adjusted the tests for gdb to test original string pretty
printing as well as pointers to them. It was doing it via the map before
but for completeness this ensures it's tested as a unit.

Change-Id: I4926547ae4fa6c85ef74301e7d96d49ba4a7b0c6
Reviewed-on: https://go-review.googlesource.com/8217
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>