[dev.power64] gc: convert Bits to a uint64 array

So far all of our architectures have had at most 32 registers,
so we've been able to use entry 0 in the Bits uint32 array
directly as a register mask.  Power64 has 64 registers, so
this converts Bits to a uint64 array so we can continue to use
entry 0 directly as a register mask on Power64.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/169060043