[dev.cc] 9a: make RET a synonym for RETURN; use "g" instead of "R30"

[dev.cc] 9a: make RET a synonym for RETURN; use "g" instead of "R30"

Previously, 9a was the only assembler that had a different
name for RET, causing unnecessary friction in simple files
that otherwise assembled on all architectures.  Add RET so
these work on 9a.

This also renames "R30" to "g" to avoid unintentionally
clobbering g in assembly code.  This parallels a change made
to 5a.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/178030043