Implement %RAW-INSTANCE-ATOMIC-INCF/WORD interpreter stub.

The vops were already working for PPC and x86 32-bit
but on 64-bit, the vop required a constant index.
But out-of-line calls didn't work on any supported platform.

Also resolve discrepancy between interpreted and compiled code:
because the macroexpansion of ATOMIC-INCF masks the 'delta'
to MOST-POSITIVE-WORD, the out-of-line code must have the identical
Lisp type as the VOP has for the primitive-type and sc restriction,
instead of the user-visible constraint of SIGNED-WORD.
Remove obsolete cautionary note from change 95a6db73.

At that time, EMIT-MAKE-LOAD-FORM did not take an optional argument
indicating to use SYMBOL-VALUE to obtain the load-time value of
a non-EQL-comparable DEFCONSTANT.
Indicate an inefficient use of PCL's function generator.