Use C11 alignas instead of palloc/malloc for alignment

There are a number of places where palloc()/malloc()/etc. was used
solely to obtain an aligned buffer. We can do these much simpler by
using alignas with a local variable instead. See attached patch.

One of these revealed a small problem with how pgindent handles alignas
(it doesn't know about it and it might or might not work well depending
on context), so I added a workaround into pgindent to fix that. (Or we
could try to reshuffle that code to avoid the problem.)