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
Also add a script for sysroot management. For now, it can only create
fake sysroots that just symlink to local folders. This is useful for
testing.
Differential Revision: https://reviews.llvm.org/D96868