This unlocks opt-in usage of BitSet64 on 32-bit platforms. It is slightly faster than IterableBitSet when the amount of bits is greater than 32.

Bug: angleproject:4473
Change-Id: I230784acc871e13b1f94c87503f6bb869dcd3a68
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2100969Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>