cmd/5l: fix encoding of new MOVB, MOVH instructions

They are just like MOVW and should be setting only
two register fields, not three.

R=ken2
CC=golang-dev, remyoudompheng
https://codereview.appspot.com/12781043
cmd/5g: factor out prog information

Like CL 12637051, but for 5g instead of 6g.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12779043
sync/atomic: specify argsize for asm routines
Fixes issue 6098.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12717043
runtime: remove unused m->racepc
The original plan was to collect allocation stacks
for all memory blocks. But it was never implemented
and it's not in near plans and it's unclear how to do it at all.

R=golang-dev, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/12724044