cmd/dist: pass correct -DGOARCH_$GOARCH flag to gcc.
cmd/dist: pass correct -DGOARCH_$GOARCH flag to gcc.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5643061

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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