Move the asm-generic/system.h xchg() implementation to asm-generic/cmpxchg.h

Move the asm-generic/system.h xchg() implementation to asm-generic/cmpxchg.h
to simplify disintegration of asm/system.h.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>