Previously compiler-rt didn't correctly passthrough cmake
variables for python when building the various santizers.
This patch passes these variables through.
This patch may also correctly apply to any version of LLVM
to any version of LLVM that uses the newer monorepo style organization,
and any older llvm newer than 7.0.0 as long as the paths were set
appropriately. However, this was not done so because it was not
tested with older LLVM releases.
Fixes#19908
See also: https://bugs.llvm.org/show_bug.cgi?id=48180
