memarea.c: use flexible array member for mem

memarea.c: use flexible array member for mem

This make clang's memory sanitizer happier that we aren't reading
off the end of a char[1].  We hadn't replaced the char[1] with a
char[FLEXIBLE_ARRAY_MEMBER] before because we were doing a union
trick to force alignment.  Now we use __attribute__(aligned) where
available, and we do the union trick elsewhere.

Most of this patch is just replacing accesses to (x)->u.mem with
(x)->U_MEM, where U_MEM is defined as "u.mem" or "mem" depending on
our implementation.