Fix CHAR-EQUAL on base-chars on non-sb-unicode.

BASE-CHAR-P, called by TWO-ARG-CHAR-EQUAL, isn't properly implemented
on non-sb-unicode, the transform by which it gets transformed into
(typep x 'base-char) is disabled, causing an infinite loop.
Since testing for base-char-p is usually redundant on #-sb-unicode,
don't define it there at all. This will catch inadvertent uses.
In the few places where it's currently used, it's can be safely
omitted.

Reported by Jan Moringen.
Micro-optimize move-immediate on x86-64.

Use XOR X, X when the immediate is 0.