With https://golang.org/cl/135455, gccgo now uses a different mangling
scheme for package paths; add code to use this new scheme for function
and variable symbols. Since users sometimes use older versions of
gccgo with newer versions of go, perform a test at runtime to see
which mangling scheme is in effect for the version of 'gccgo' in the
path.

Updates #27534.

Change-Id: If7ecab06a72e1361129fe40ca6582070a3e8e737
Reviewed-on: https://go-review.googlesource.com/c/144418Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>