[gdb/testsuite] Add xfail for PR gcc/101643

[gdb/testsuite] Add xfail for PR gcc/101643

With gcc 8.5.0 I run into:
...
(gdb) print bad^M
$2 = (0 => 0 <repeats 25 times>)^M
(gdb) FAIL: gdb.ada/big_packed_array.exp: scenario=minimal: print bad
...
while with gcc 9.3.1 we have instead:
...
(gdb) print bad^M
$2 = (false <repeats 196 times>)^M
(gdb) PASS: gdb.ada/big_packed_array.exp: scenario=minimal: print bad
...

This is caused by gcc PR, which I've filed at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101643 "[debug, ada] packed array
not described as packed".

Fix by marking this as XFAIL.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-07-27  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26904
	* gdb/testsuite/gdb.ada/big_packed_array.exp: Add xfail.