[builtins] Build for arm64_32 for watchOS (Darwin)

Trying to build the builtins code fails because `arm64_32_SOURCES` is
missing. Setting it to the same list used for `aarch64_SOURCES` solves
that problem and allow the builtins to compile for that architecture.

Additionally, arm64_32 is added as a possible architecture for watchos
platforms.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D99690