ARM: cmpxchg: avoid warnings from macro-ized cmpxchg() implementations

A recent change in kernel/acct.c added a new warning for many
configurations on ARM:

kernel/acct.c: In function 'acct_pin_kill':
arch/arm/include/asm/cmpxchg.h:122:3: warning: value computed is not used [-Wunused-value]

The code is in fact correct, it's just a cmpxchg() call that
intentionally ignores the result, and no other code does that.  The
warning does not show up on x86 because of the way that its cmpxchg()
macro is written. This changes the ARM implementation to use a similar
construct with a compound expression instead of a typecast, which causes
the compiler to not complain about an unused result.

Fix the other macros in this file in a similar way, and place them
just below their function implementations.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>