more backports for 1.9.2

BUG: Fixes ndarray.fill to accept maximum uint64 value

Fix too large alignment of complex types

The offending commit is c53b0e4, which introduced two regressions:

- using astype to cast a structured array to one with a different byte
  order no longer works;
- comparing structured-array dtypes can give incorrect results if the
  two dtypes have different byte orders.

This pull request should fix both.

One thing I wasn't sure about is reordering struct fields.  In my
implementation, the `equiv`, `same_kind`, and `safe` rules are now
allowed to reorder fields.  If that isn't desired, though, it's a pretty
easy change.

If itemsize is a power of two use that as the required alignment up to
the maximum provided by the platform. Power of two sizes may be accessed
via larger moves than bytes.
Non-power of two sizes are accessed bytewise and can thus always be
considered aligned.
Closes gh-5293

TST: added test for PyArray_AsCArray #5313