The FILE structure has a mbstate_t in it. This structure needs to be
aligned on a int64_t boundary. However, when we allocate the array of
these structures, we use ALIGNBYTES which defaults to sizeof(int) on
arm, i386 and others. The i386 stuff can handle unaligned accesses
seemlessly. However, arm cannot. Take this into account when creating
the array of FILEs, and add some comments about why.

Differential Revision: https://reviews.freebsd.org/D4708