Pass -c to generate an object.  Pass GOPKGPATH as a symbol, not a
string.  Pass -xassembler-with-cpp so that the preprocessor is run.

Change-Id: I84690a73cc580bb05724ed07c120cec9cfd5e48b
Reviewed-on: https://go-review.googlesource.com/18733
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>