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/atomic.h, and it's impractical
to convert the header 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 atomic_*() or arch_atomic_*()
depending on whether ARCH_ATOMIC is selected. To do this, the C
implementations are prefixed as generic_atomic_*(), and preprocessor
definitions map atomic_*()/arch_atomic_*() 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/20210525140232.53872-10-mark.rutland@arm.com