ARM: Emulate one-byte cmpxchg

ARM: Emulate one-byte cmpxchg

Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on ARM systems
with ARCH == ARMv6K.

[ paulmck: Apply Arnd Bergmann and Nathan Chancellor feedback. ]
[ paulmck: Apply Linus Walleij feedback. ]

Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/54798f68-48f7-4c65-9cba-47c0bf175143@sirena.org.uk/
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/all/CA+G9fYuZ+pf6p8AXMZWtdFtX-gbG8HMaBKp=XbxcdzA_QeLkxQ@mail.gmail.com/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Davis <afd@ti.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Eric DeVolder <eric.devolder@oracle.com>
Cc: Rob Herring <robh@kernel.org>
Cc: <linux-arm-kernel@lists.infradead.org>