When cross compiling we are forcing the usage of the resource directory from
the bootstrapping0 compiler to ensure we are picking up the stdlib from
the SDK.
As a result we need to ensure to have legacy layouts for all the
architectures in that folder.
Addresses rdar://98899578
Adding build modes for libswift: off, hosttools, bootstrapping, bootstrapping-with-hostlibs
The two bootstrapping modes are new. For details see libswift/README.md
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.
