exp/types: don't test importing types if using gccgo
exp/types: don't test importing types if using gccgo

The exp/types packages does not support the gccgo export data
format.  At some point it should, but not yet.

R=gri, bradfitz, r, iant, dsymonds
CC=golang-dev
http://codereview.appspot.com/6854068

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