Updates cgo's gccgoPkgpathToSymbolNew() to bring it into
conformance with the way that gccgo now handles package
paths with embedded dots (see CL 200838). See also
https://gcc.gnu.org/PR61880, a related bug. This CL is a
copy of CL 207957 in the main Go repo.
Updates golang/go#35623.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/207977
From-SVN: r278470
