gc: fix arm build

gc: fix arm build

Escape analysis was incorrectly assuming that
functions without bodies don't leak their
parameters.  This meant that sync/atomic's
TestAddInt64 was allocating x on its stack,
and then x was not properly aligned for use
with the atomic 64-bit instructions.  Obviously
we should figure out the alignment story on 5g
too, but this fix is correct and should restore the
build to 'ok'.

TBR=lvd
CC=golang-dev
https://golang.org/cl/4964047