ia64: populate the cmpxchg header with appropriate code

commit 93f37888

    "Fix ia64 build errors (fallout from system.h disintegration)"

introduced arch/ia64/include/asm/cmpxchg.h as a temporary
build fix and stated:

    "... leave the migration of xchg() and cmpxchg() to this new
     header file for a future patch."

Migrate the appropriate chunks from asm/intrinsics.h and fix
the whitespace issues in the migrated chunk.

Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: David Howells <dhowells@redhat.com>
Acked-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>