Add a kernel with most polling devices removed and HZ reduced to ease
benchmarking.  This is similar to the kernel used in the ISCA submission,
but in a single file.
Style: quote "no" in test aginst ${MK_CHERI}
Simplify CHERI Makefiles.

Makefiles for software using CHERI capabilties may now define USE_CHERI
to override CC, CFLAGS, and LDFLAGS.

If any such Makefile is part of a build CHERI_CC must be defined.