gc: unify stack frame layout

gc: unify stack frame layout

allocparams + tempname + compactframe
all knew about how to place stack variables.

Now only compactframe, renamed to allocauto,
does the work.  Until the last minute, each PAUTO
variable is in its own space and has xoffset == 0.

This might break 5g.  I get failures in concurrent
code running under qemu and I can't tell whether
it's 5g's fault or qemu's.  We'll see what the real
ARM builders say.

R=ken2
CC=golang-dev
https://golang.org/cl/4973057