alpha: no need to include asm/xchg.h twice

We used to generate different helpers for local and full
{cmp,}xchg(); these days the barriers are in arch_{cmp,}xchg()
instead and generated helpers are identical for local and
full cases.  No need for those parametrized includes of
asm/xchg.h - we might as well insert its contents directly
in asm/cmpxchg.h and do it only once.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>