arm: work around reg allocator bug in 5g, in two parts.

arm: work around reg allocator bug in 5g, in two parts.
1) hack regalloc to leave R9 (m) and R10 (g) alone.
the real fix is tricker, but this gets us running
2) fix up the few places in the package sources that
the shortage of registers affects, by simplifying
some expressions.

all of this should be reverted when the right fix is in.

Fixes #1084.

R=rsc
CC=golang-dev
https://golang.org/cl/2132046