SEQUENCE:PROTOCOL-UNIMPLEMENTED references node in SBCL manual
A few more changes to internal error numbering.

- It should not be the case that C/assembly runtime needs to know all
  error numbers. Stop exposing most of the constants, to lessen potential
  confusion/conflict on things like CONTROL_STACK_EXHAUSTED_ERROR,
  which is a static symbol that falsely appears to occupy the same realm
  as the small integer <x>_ERROR constants.

- Stop dumping huge s-expressions for the type that describes
  all specialized simple arrays of rank 1.

- Rename OBJECT-NOT-FUN to OBJECT-NOT-FUNCTION.
Comment all widetag values correctly for 32- and 64-bit

... rather than dismissively say that they get screwy.
Improve NUMBERP and REALP on 64-bit architectures.
Redo previous patch to not make FLOATP and SINGLE-FLOAT-P worse.
Add another internal error number presumed to have existed.
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
Add more #+sb-doc.
Fix a typo.
Dummy in basics for ARM port.

  * Added commentary for :ARM feature to base-target-features.

  * Altered feature sanity-checker to know about ARM and believe
that it is a cheneygc target (can and probably will be changed
later, but a cheneygc bringup should be easier than a gencgc
bringup).

  * Added an :ARM target mapping for source files.

  * Added dummy files for each target-specific source file in the
build process.
KLUDGE tfb/ldso-stubs.lisp to at least build for ARM.

  * This is an outright KLUDGE, and will not produce output that
survives make-target-1.  It will, however, build, and that's
sufficient to allow hacking to proceed for make-host-1.

  * At some point in the future, once the correct LDSO_STUBIFY
magic has been determined, this commit should probably be amended.
compiler/arm/parms.lisp: Lay in the most basic machine info.

  * This amounts to describing how wide a heap word is, how wide a
machine register is, and so on.

  * Also define some assembler-related parameter, for no really
good reason.

  * All constants copied from the PPC backend.