x86: Fix LOCK_PREFIX_HERE for uniprocessor build

Checkin b3ac891b:
x86: Add support for lock prefix in alternatives

... did not define LOCK_PREFIX_HERE in the case of a uniprocessor
build.  As a result, it would cause any of the usages of this macro to
fail on a uniprocessor build.  Fix this by defining LOCK_PREFIX_HERE
as a null string.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Luca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267005265-27958-2-git-send-email-luca@luca-barbieri.com>