libgo: fix DejaGNU testsuite compiler when using build sysroot

Patch by Maciej W. Rozycki.

Change-Id: I039d0977bc19744f654c076fc75a98ad5f57cf7a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/206577
Reviewed-by: Ian Lance Taylor <iant@golang.org>