Round up the size of byval arguments to MinAlign

Otherwise we can end up with an argument frame size that is not a
multiple of stack slot size, which is very awkward.

This fixes PR20547, which was a bug in x86_64 Sys V vararg handling.
However, it's much easier to test this with x86 callee-cleanup
functions, which previously ended in "retl $6" instead of "retl $8".

This does affect behavior of all backends, but it presumably fixes the
same bug in all of them.