sysroot.py: add support for non-darwin platforms

CMAKE_SYSROOT works fine here, and `sysroot.py make-fake`
borders on trivial here, but I suppose it's still nice
to have a consistent script to set these up across platforms.

And these are the platforms where we can do real sysroot management one
day.

Differential Revision: https://reviews.llvm.org/D96882