aarch64 windows build failure

[theKidOfArcrania]

There is a bug when trying to build for windows arm64 target. See in line 145 concatenates a path of env $OUT_DIR to "/outlined_atomics.rs". The problem arises in windows because $OUT_DIR may resolve to an unc path in windows, and those do not support having forward slashes. Using something like Path.join(...) should fix this bug.

[Amanieu]

I don't have a setup for building arm64 windows. Could you submit a PR with the fix once you have tested it?