cmd/5g, etc: prepare to unexport gc.Mp*

cmd/5g, etc: prepare to unexport gc.Mp*

Remove all uses of Mp* outside of the gc package.

A subsequent, automated commit in the Go 1.6
cycle will unexport all Mp* functions and types.

No functional changes. Passes toolstash -cmp.

Change-Id: Ie1604cb5b84ffb30b47f4777d4235570f2c62709
Reviewed-on: https://go-review.googlesource.com/9263
Reviewed-by: Russ Cox <rsc@golang.org>