cmd/go: if there are C++ sources, use g++ as default external linker

This will bring in the C++ standard library without requiring
any special #cgo LDFLAGS options.

When using gccgo, just add -lstdc++ to link line; this should
do no harm if it is not needed.

No tests, since we don't want to assume a C++ compiler.

Update issue 5629

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/13394045