Go patch committed: Let test import package that imports tested pkg

[Ian Lance Taylor]

This patch to gccgo lets a test of package P1 import a package P2 where
P2 itself imports P1. gccgo was giving an error for this kind of
import. That can be useful in detecting a failure to rebuild a package
that depends on some other package, but it prevented this useful
behaviour. So I removed the check. This in turn let me revert some
gccgo-specific changes to libgo. Bootstrapped and ran Go testsuite on
x86_64-unknown-linux-gnu. Committed to mainline.

Ian