asm-generic/bitops: Update stale comment

The comment in 'asm-generic/bitops.h' states that you should "recode
these in the native assembly language, if at all possible". This is
pretty crappy advice now that the generic implementation is defined in
terms of atomic_long_t rather than a spinlock, so update the comment and
hopefully save future architecture maintainers a bit of work.

Reported-by: Stefan Asserhall <stefana@xilinx.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200213093927.1836-1-will@kernel.org