powerpc/64s: Consolidate exception handler alignment

Move exception handler alignment directives into the head-64.h macros,
beause they will no longer work in-place after the next patch. This
slightly changes functions that have alignments applied and therefore
code generation, which is why it was not done initially (see earlier
patch).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>