[openmp] [llvm] [openmp][wasm] Allow compiling OpenMP to WebAssembly (PR #71297)

[Andrew Brown via llvm-commits]

abrown wrote:

Ok, I rebased this on `main`, which fixes the conflicts due to a recent addition of the s390x target. I'm open to splitting this up into separate PRs but I'll wait until it's more clear who the primary reviewer is and what other feedback I should address.

https://github.com/llvm/llvm-project/pull/71297