Make static functions interrupt safe on ARM.

Don't put things onto the control stack before the stack pointer isn't
written back.
Fix PA handling in call_into_x on ARM.

The pseudo-atomic flag was untagged into the wrong side, LSL instead
of LSR.
compiler/arm/call: Fix minimum full-call-frame size.

  * We need to allocate a minimum of two words (up from the one
that we were originally allocating), one for the frame link, and
one for the LRA.  Without allocating this second word, the LRA is
subject to being overwritten during an inconveniently-timed
interrupt.
code/debug-int,runtime/arm-assem.S: Fix undefined-function backtrace.

  * Two things here.  First is that MAKE-LISP-OBJ didn't recognize
undefined_tramp as a valid object.  Easily enough fixed.  Second
is that the undefined_tramp wasn't initializing reg_CODE before
trapping, leaving the debugger looking in the wrong place for the
error trap location.  Also easily enough fixed.

  * Unlike PPC, we always have fully-initialized call frames
constructed on the caller side, so we don't need to do the rest of
the fixups that PPC does.