Fix big-endian aggregate assignment in Ada

Fix big-endian aggregate assignment in Ada

A bug internal to AdaCore notes that assigning a non-scalar value to
an element of a packed array will sometimes fail.

The bug turns out to be that ada_value_assign incorrectly computes the
starting point for the assignment.  This patch fixes the problem.

gdb/ChangeLog
2019-05-01  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_value_assign): Correctly compute starting offset
	for big-endian copies.

gdb/testsuite/ChangeLog
2019-05-01  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/packed_array_assign.exp: Add packed assignment
	regression test.