graphics/mesa-dri: fix builds against llvm 20 and 21

Add several upstream patches, as patch-llvm-20-* and patch-llvm-21-*, to
make mesa build against llvm 20 and 21. Whenever mesa gets updated to a
new version which includes those changes, the patches can easily be
removed.

PR: 290809, 290874
MFH: 2026Q2
(cherry picked from commit e45b1bd3027b31845938ab6c7a5adc28c87d7ece)