Add the KMSAN runtime

KMSAN enables the use of LLVM's MemorySanitizer in the kernel. This
enables precise detection of uses of uninitialized memory. As with
KASAN, this feature has substantial runtime overhead and is intended to
be used as part of some automated testing regime.

The runtime maintains a pair of shadow maps. One is used to track the
state of memory in the kernel map at bit-granularity: a bit in the
kernel map is initialized when the corresponding shadow bit is clear,
and is uninitialized otherwise. The second shadow map stores
information about the origin of uninitialized regions of the kernel map,
simplifying debugging.

KMSAN relies on being able to intercept certain functions which cannot
be instrumented by the compiler. KMSAN thus implements interceptors
which manually update shadow state and in some cases explicitly check
for uninitialized bytes. For instance, all calls to copyout() are
subject to such checks.

The runtime exports several functions which can be used to verify the
shadow map for a given buffer. Helpers provide the same functionality
for a few structures commonly used for I/O, such as CAM CCBs, BIOs and
mbufs. These are handy when debugging a KMSAN report whose
proximate and root causes are far away from each other.

Obtained from: NetBSD
Sponsored by: The FreeBSD Foundation