Thou shalt not MAKE-OTHER-IMMEDIATE-TYPE.

Thou shalt not MAKE-OTHER-IMMEDIATE-TYPE.

  * Really, it's only ever used to make unbound-markers, for which
we have MAKE-UNBOUND-MARKER, which produces more optimal code in
the first place, has a shorter invocation, and reveals intention
far better than using MAKE-OTHER-IMMEDIATE-TYPE.

  * Rewrite all uses of MAKE-OTHER-IMMEDIATE-TYPE, excise the VOPs
from all of the backends, and remove the symbol name from package-
data.

  * And add DESCRIPTOR-REG to the permitted SCs for
MAKE-UNBOUND-MARKER, since it now must be legal to use a TN of
primitive-type T.