Fix bit strides for -fgnat-encodings=minimal

With -fgnat-encodings=minimal, the enum_idx_packed.exp test will fail.
In this test case, we have an array (with dynamic length) of arrays,
and the inner array has a bit stride.  In this situation, the outer
array's bit stride must be updated to account for the entire bit
length of the inner array.

Here, again, some tests must be kfail'd when an older version of GNAT
is in use.

gdb/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdbtypes.c (update_static_array_size): Handle bit stride.

gdb/testsuite/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/enum_idx_packed.exp: Test two forms of -fgnat-encodings.