ARM: v6k: select cmpxchg code sequences according to V6 variants

If CONFIG_CPU_V6 is enabled, we must avoid the byte/halfword/doubleword
exclusive operations, which aren't implemented before V6K.  Use the
generic versions (or omit them) instead.

If CONFIG_CPU_V6 is not set, but CONFIG_CPU_32v6K is enabled, we have
the K extnesions, so use these new instructions.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>