{compiler,assembly}/arm/: Treat CSP-TN as if it were a memory location.

  * In preparation for making CSP-TN our number-stack pointer (for
reasons of interrupt reentrancy), introduce a couple of macros to
paper over its actual storage location and arrange never to refer
to it directly in assembly-routines or in code generated by the
compiler.
Move ARM control stack pointer to a static-symbol.

  * The simplest part of this whole mess is in src/compiler/arm/,
where we define a couple of new static-symbols (one for the
runtime to use during interrupt processing) and we switch out the
LOAD-CSP and STORE-CSP macros in favor of using a symbol value.

  * The next tricky bit is in interrupt.c, where we change how we
access the control stack pointer on ARM, and where we arrange to
store the control stack pointer where a non-local unwind won't
screw up if it runs across it but still where it can be restored
if we do a "normal" exit (necessary because it's no longer part of
the signal context).

  * And the really tricky bit is fixing up call_into_lisp and
call_into_c to operate in terms of the new symbol instead of in
terms of the old register.  Hopefully I got that right, and it's
about to change again, anyway.
Move ARM number stack pointer to r14.

  * This is straightforward, but touches a lot of places.

  * On the compiler side, rename CSP to NSP, remove the static
symbol *NUMBER-STACK-POINTER*, and fix up all of the number stack
manipulation to use the register instead of the symbol.

  * On the runtime side, fix up the register name in arm-lispregs,
and strip out all of the crazy stack games in arm-assem.
runtime/interrupt.c: ARM no longer needs the altstack.

  * We'll leave the setup code in place for now, but disable the
switching of stacks when we register each signal handler.
Properly encode immediates in [DE]ALLOC-NUMBER-STACK-SPACE on ARM.

Large allocation sizes may not be encodable as immediates, use
composite-immediate-instruction.