locking/atomic/x86: Introduce arch_atomic64_read_nonatomic() to x86_32

Introduce arch_atomic64_read_nonatomic() for 32-bit targets to load
the value from atomic64_t location in a non-atomic way. This
function is intended to be used in cases where a subsequent atomic
operation will handle the torn value, and can be used to prime the
first iteration of unconditional try_cmpxchg() loops.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240410062957.322614-2-ubizjak@gmail.com