Improve the way we disable audit tests

This diff should likely be revisited, but for now check that we're
targeting a CPU that supports CHERI rather than that we're building
optional CHERI programs.

With this change, MK_CHERI only controls optional programs that require
CHERI and aren't likely to be upstreamed in a CHERI merge.

tests/sys/Makefile

@@ -42,7 +42,7 @@ TESTS_SUBDIRS+= vmm
 # during cleanup. They start and stop a new auditd for every test CASE which
 # takes a really long time on QEMU. In the latest jenkins run these tests took
 # 9.5 hours to run! Skip them until we can run them at a reasonable speed.
-.if ${MK_AUDIT} != "no" && ${MK_CHERI} == "no"
+.if ${MK_AUDIT} != "no" && ${MACHINE_CPU:Mcheri}
 _audit= audit
 .endif