asm-generic: cmpxchg does not handle non-long arguments

The version of cmpxchg defined in asm-generic/system.h does not handle
correctly non-long arguments. Use the version defined in cmpxchg.h
instead.

Signed-off-by: Mathieu Lacage <mathieu.lacage@inria.fr>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>