llvm19: permit incremental builds from llvm18

Detect and delete out of date clang-tblgen produced output. I found it
sufficent for compilation to delete only arm_*.inc files, but it seems
safer to regenerate all output and a few extra clang-tblgen runs pales
in comparison to the full build we're doing at this point.

Reviewed by: dim, emaste
Differential Revision: https://reviews.freebsd.org/D47270