cmd/5c, cmd/5g, cmd/5l: introduce MOVBS and MOVHS instructions.

MOVBS and MOVHS are defined as duplicates of MOVB and MOVH,
and perform sign-extension moving.
No change is made to code generation.

Update issue 1837

R=rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/12682043