Book-E/AIM exec bits unification

[Nathan Whitehorn]

As part of the ppc64 work, which I am starting to merge, I ended up
unifying non-machine-dependent but ABI-dependent bits of machdep.c and
vm_machdep.c into a new generic powerpc/exec_machdep.c file. This
reduces some code duplication and should ease implementation of
Book-E systems with hard FP, Altivec, and 64-bit support. A diff to do
this is at http://people.freebsd.org/~nwhitehorn/exec_machdep.diff

I have tested this extensively on AIM, since it's been part of the
ppc64 tree for a long time now, but it is only compile-tested on Book-E
systems, so I'd appreciate knowing if I broke anything there. Unless I
hear anything, I'll commit this to HEAD on Monday.
-Nathan