bsd.cpu.mk: add support for x86-64-v[234] levels as CPUTYPE

These are supported in LLVM 12 and GCC 11:

https://github.com/llvm/llvm-project/commit/012dd42e027e
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=324bec558e95584e8c1997575ae9d75978af59f1

Differential Revision: https://reviews.freebsd.org/D29150

(cherry picked from commit 362fef2f4a9954c0f5eeecbe512cc5abbdc44263)