[dev.power64] runtime: make all bitmaps arrays of bytes

The "simpler faster garbage collector" is full of little-endian assumptions.
Instead of trying to correct all the mistakes, just give in and make
everything use bytes.

LGTM=minux
R=minux
CC=dvyukov, golang-codereviews
https://codereview.appspot.com/124400043
[dev.power64] runtime: fix usleep not to smash saved LR

LGTM=minux
R=minux
CC=golang-codereviews
https://codereview.appspot.com/130760043