FoundationMacros: use cross-compilation to build for host (#714)

[compnerd]

Use ExternalProject to switch FoundationMacros to cross-compilation. This allows us to build the macros for the right OS/architecture when cross-compiling Foundation for other environments.

[compnerd]

Explanation: Build changes necessary to support building macros for Windows.
Scope: Enables building and distributing the Foundation macros on all platforms.
Issue:
Original PR: #714
Risk: We would need additional changes to build and distribute the macros.
Testing: The original changes were tested as part of building the toolchain and the installer for Windows as well as the built image for Linux.
Reviewer: @jmschonfeld

[compnerd]

@swift-ci please test

[jmschonfeld]

Backporting this sounds like a good idea to me, let's just make sure that's ok with @etcwilde since his Musl PR also tries to address this, and let's also make sure we back port the swift repo changes (for build-script and build.ps1) and the installer script changes to install the macro on Windows

[etcwilde]

If we're waiting on me for approval, I'm fine with this. I just need a decision landed so that I can get the static SDK tipped back upright.