mesa*: Update to 21.1.5
- swrast is only a gallium driver now.
- Bump needed llvm to 12
- libclc is still a llvm10, moving to >10 needs to change from where we pick libclc as its now part of llvm
Differential Revision: https://reviews.freebsd.org/D31165
Reviewed by: kbowling, zeising