parisc: fix missing cmpxchg file error from system.h split

Commit b4816afa ("Move the asm-generic/system.h xchg()
implementation to asm-generic/cmpxchg.h") introduced the concept of
asm/cmpxchg.h but the parisc arch never got one.  Fork the cmpxchg
content out of the asm/atomic.h file to create one.

Some minor whitespace fixups were done on the block of code that created
the new file.

Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Helge Deller <deller@gmx.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>