lib: Only test atomic64_dec_if_positive on archs having it

Currently atomic64_dec_if_positive() is only supported by PowerPC,
MIPS and x86-32.

Signed-off-by: Luca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267183361-20775-1-git-send-email-luca@luca-barbieri.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>