Update RawTherapee to 5.6 release.
Announcement: https://rawtherapee.com/blog/rawtherapee-5.6-released
Only offer OPENMP option on x86/amd64 for now.
While here, avoid -mtune=generic on anything but x86/amd64/aarch64. [1]
PR: 237285 [1]
Submitted by: pkubaj [1]