Three microoptimizations to numeric tag testing for 1-bit fixnum tag.

Each of the following might save 1 byte on x86-64, depending on
register allocation.

- Instead of "LEA EAX, [Rxx - other-pointer-lowtag]" use full-width RAX.
  At worst this adds a REX prefix, but might not if Rxx needed it anyway.
  The payback is in loading widetag from BYTE PTR [RAX].
  After the LEA we never need a displacement, which either breaks even
  by saving 1 byte, or wins - nets a 1 byte savings if Rxx had required
  a REX prefix.
  Note: RAX is an alias pointer, but safe because of GC conservativism.

These two depend on AL having the shortest encoding:
- Do the fixnum lowtag test only after loading RAX by checking for
  a 1 bit instead of looking for a 0 bit in the original value.
- Similarly for SHORT-FLOAT, do the immediate tag test on AL
  compensating for the biased value in the widetag field.
Fix a C compiler warning about discarding 'volatile' qualifier

Initially I did this by explicitly casting ret to (void*)
but I showed that to another hacked who disapproved because:
 - dlopen() does not return (volatile void*)
 - 'ret' need not exist at all, at least not without an
   explanatory comment by its original author
Produce marginally less bad code for %FUN-FUN
Tweak x86-64 signed-byte-64-p in the same way as other type-vops.
Undo optimization that Stas didn't like

This still manages to improve REALP and NUMBERP by using AL
for the fixnum tag test, short-float test, and lowtag test;
but uses the original tagged register to load widetag.
DEFCONSTANT should remove a symbol-macro expansion
Apply the same bandaid as Gabor Melis did in changeset 6208e9
Silence /cold-fdefinition-object when compiling with sb-show

Most of the calls to show in 'genesis' would be better off using
a totally different macro anyway, since it's all host stuff.
Remove dead code for purify of compact-info-environment