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

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

Commit b4816afa3986 ("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" <[email protected]>
Cc: Helge Deller <[email protected]>
Signed-off-by: Paul Gortmaker <[email protected]>
Acked-by: David Howells <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>