Stack realignment

Hello, Everyone.

I've just checked code for dynamic stack realignment. This is needed on
targets, where ABI-defined stack alignment is not so big (e.g.
x86-32/linux, mingw32, cygwin, etc), but some stack objects (e.g.
vectors) will require higher alignment.

I think, I checked all 'hard' situations and code also survived llvm-gcc
bootstrap, Qt, Mozilla, Manta compilation, but please carefully check
all vector-used routines on such targets (especially if they stopped to
work recently) and fill out PRs if something will go wrong.