cmd/cgo: fix typo in gccgo name mangling recipe

The code to implement new-style gccgo name mangling had a recipe that
didn't quite match that of the compiler (incorrect handling for '.').
This showed up as a failure in the gotools cgo test if the directory
containing the test run included a "." character.

[This is a copy of https://golang.org/cl/147917].

Change-Id: Ia94728ecead879c8d223eb6cee6c102a8af1c86e
Reviewed-on: https://go-review.googlesource.com/c/147937


Reviewed-by: Cherry Zhang <cherryyz@google.com>