cmd/cgo: better handling for '.' in pkgpath for gccgo

Update gccgoPkgpathToSymbolNew() to bring it into conformance
with the way that gccgo now handles packagepaths with embedded
dots (see CL 200838). See also https://gcc.gnu.org/PR61880, a
related bug.

Updates #35623.

Change-Id: I32f064320b9af387fc17771530c745a9e3003c20
Reviewed-on: https://go-review.googlesource.com/c/go/+/207957


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>