sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Only define sanitizer wrappers for unsized bus space operations once.

Previously, this was defining duplicate definitions for each size.

This fixes a redundant definition warning from GCC.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D31966

Simplify kernel sanitizer interceptors

KASAN and KCSAN implement interceptors for various primitive operations
that are not instrumented by the compiler. KMSAN requires them as well.
Rather than adding new cases for each sanitizer which requires
interceptors, implement the following protocol:
- When interceptor definitions are required, define
SAN_NEEDS_INTERCEPTORS and SANITIZER_INTERCEPTOR_PREFIX.
- In headers that declare functions which need to be intercepted by a
sanitizer runtime, use SANITIZER_INTERCEPTOR_PREFIX to provide
declarations.
- When SAN_RUNTIME is defined, do not redefine the names of intercepted
functions. This is typically the case in files which implement
sanitizer runtimes but is also needed in, for example, files which
define ifunc selectors for intercepted operations.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation

Generalize bus_space(9) and atomic(9) sanitizer interceptors

Make it easy to define interceptors for new sanitizer runtimes, rather
than assuming KCSAN. Lay a bit of groundwork for KASAN and KMSAN.

When a sanitizer is compiled in, atomic(9) and bus_space(9) definitions
in atomic_san.h are used by default instead of the inline
implementations in the platform's atomic.h. These definitions are
implemented in the sanitizer runtime, which includes
machine/{atomic,bus}.h with SAN_RUNTIME defined to pull in the actual
implementations.

No functional change intended.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation

Rename _cscan_atomic.h and _cscan_bus.h to atomic_san.h and bus_san.h

Other kernel sanitizers (KMSAN, KASAN) require interceptors as well, so
put these in a more generic place as a step towards importing the other
sanitizers.

No functional change intended.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29103