Also introduce BGET2/4, BPUT2/4 as they are widely used.
Slightly improve BGETC/BPUTC implementation.
This gives ~5% CPU time improvement on go install -a -p1 std.
Before:
real		user		sys
0m23.561s	0m16.625s	0m5.848s
0m23.766s	0m16.624s	0m5.846s
0m23.742s	0m16.621s	0m5.868s
after:
0m22.999s	0m15.841s	0m5.889s
0m22.845s	0m15.808s	0m5.850s
0m22.889s	0m15.832s	0m5.848s

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12745047