Cirrus-CI: switch to Clang/LLVM 12

dim@ is preparing to import Clang/LLVM 12 into the base system as the
system compiler / toolchain. Apply the same change to the Cirrus-CI
config.

Reviewed by: imp, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30109