[llvm] [Support][Memory] Add memfd based fallback for strict W^X Linux systems (PR #98538)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> I did a review and fixes myself and added them to the branch as follol-up commits, to ease review. My plan is to squash these into the first commit for a merge, though.

llvm only allows the "squash and merge" option, so yes, you're doing the right thing :)

https://github.com/llvm/llvm-project/pull/98538