GHA 6 hours limit, intel runners

[Paul Guyot]

Hello,

Should we try to circumvent GitHub actions 6 hours limit?
The problem arises with clang updates that time out, but these timeouts may hide issues with flang.

Alternatively we could split the llvm subports into separate files so CI only rebuilds what was updated.

I tried a prototype of waves, but I don't think it's very pretty:
https://github.com/pguyot/macports-ports/actions/workflows/wave-build.yml

Also should we also have intel runners on GHA? I wish we had a build bot so that intel Tahoe binaries were available, but at least we can ensure everything compiles on Intel?

Paul