Commit the platform definition and build script work necessary to
cross-compile for arm64_32.
arm64_32 is a variant of AARCH64 that supports an ILP32 architecture.
Bitsets implement sorted sets over nonnegative integers up to a predetermined maximum value.
These are intended to replace _UnsafeBitMap. The latter will be removed once its usages are eliminated.
- _UnsafeBitset.Word is the underlying abstraction, implementing a bitset using a single UInt value.
- _UnsafeBitset is a view over a contiguous range of words.
- _Bitset is a COW value type implementing the same construct.
