ARM/Dwarf: correctly align stack before callee-saved VPRs

We were making an attempt to do this by adding an extra callee-saved GPR (so
that there was an even number in the list), but when that failed we went ahead
and pushed anyway.

This had a couple of potential issues:
  + The .cfi directives we emit misplaced dN because they were based on
    PrologEpilogInserter's calculation.
  + Unaligned stores can be less efficient.
  + Unaligned stores can actually fault (likely only an issue in niche cases,
    but possible).

This adds a final explicit stack adjustment if all other options fail, so that
the actual locations of the registers match up with where they should be.
ARM: try to add extra CS-register whenever stack alignment >= 8.

We currently try to push an even number of registers to preserve 8-byte
alignment during a function's prologue, but only when the stack alignment is
prcisely 8. Many of the reasons for doing it apply also when that alignment > 8
(the extra store is often free, and can save another stack adjustment, though
less frequently for 16-byte stack alignment).