libgo: handle --with-toolexeclibdir=.

Patch by Maciej W. Rozycki.

Change-Id: Ifeeae4e164f4e5bc7039faffea20b993add820d7
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/216239
Reviewed-by: Ian Lance Taylor <iant@golang.org>
5 files changed