[libcxx] [docs] Update docs about how to build for Windows

Refresh the existing paragraphs on building in MSVC configurations,
add a sample of one working configuration for MinGW, and add more
details on what's necessary to run the tests these days.

Differential Revision: https://reviews.llvm.org/D97166

GitOrigin-RevId: 995a128f07b60e82fdf7eb1e13670b4cda992379
2 files changed