We'd like all architectures to convert to ARCH_ATOMIC, as this will
enable functionality, and once all architectures are converted it will
be possible to make significant cleanups to the atomic headers.

A number of architectures use asm-generic/cmpxchg.h or
asm-generic/cmpxhg-local.h, and it's impractical to convert the headers
and all these architectures in one go. To make it possible to convert
them one-by-one, let's make the asm-generic implementation function as
either cmpxchg*() or arch_cmpxchg*() depending on whether ARCH_ATOMIC is
selected. To do this, the generic implementations are prefixed as
generic_cmpxchg_*(), and preprocessor definitions map
cmpxchg_*()/arch_cmpxchg_*() onto these as appropriate.

Once all users are moved over to ARCH_ATOMIC the ifdeffery in the header
can be simplified and/or removed entirely.

For existing users (none of which select ARCH_ATOMIC), there should be
no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210525140232.53872-13-mark.rutland@arm.com