[dev.cc] cmd/9c: remove

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/175940043
[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://codereview.appspot.com/178030043
[dev.cc] runtime: convert power64 signal handlers from C to Go

The power64 equivalent of CL 168500044

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/175280043
[dev.cc] runtime: convert power64 assembly files for C to Go transition

The power64 equivalent of CL 168510043

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/178940043
[dev.cc] runtime: convert power64-specific .c and .h files to Go

The power64 equivalent of CL 174860043

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/179890043
[dev.cc] runtime: catch defs_linux_power64*.go up to other archs

Fix a constant conversion error.  Add set_{sec,nsec} for
timespec and set_usec for timeval.  Fix type of
sigaltstackt.ss_size.

LGTM=rsc
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/180840043