Allow inlining more calls to INVOKE-WITH-SAVED-FP-AND-PC during XC.

  * The INVOKE-WITH-SAVED-FP-AND-PC mechanism was defined in
ALIENCOMP, which occurs well after the first uses of ALIEN-FUNCALL,
thus preventing it from being inlined when used during XC (by
default, only on x86).

  * Fix, by relocating the mechanism from SB!C to
SB!ALIEN-INTERNALS and from COMPILER;ALIENCOMP to
CODE;EARLY-ALIENEVAL.

  * Also relocate and publish symbols for all of the magic from
SB!ALIEN-INTERNALS.
code/interr: Hook internal error contexts into the saved-fp-and-pc mechanism.

  * This covers the unfortunate case of a signal handler not
having an unbroken stack frame chain to the interrupted context,
which actually occurs on threaded x86-64 FreeBSD systems.

  * Use the existing saved-fp-and-pc mechanism, used for
ALIEN-FUNCALL to cover for code compiled -fomit-frame-pointer to
treat the internal error context as an alien funcall point.