[gdb/testsuite] Fix gdb.ada/big_packed_array.exp xfail for -m32

[gdb/testsuite] Fix gdb.ada/big_packed_array.exp xfail for -m32

With test-case gdb.ada/big_packed_array.exp and target board unix/-m32 I run
into:
...
(gdb) print bad^M
$2 = (0 => 0 <repeats 24 times>, 160)^M
(gdb) FAIL: gdb.ada/big_packed_array.exp: scenario=minimal: print bad
...

The problem is that while the variable is an array of 196 bits (== 24.5 bytes),
the debug information describes it as 25 unsigned char.  This is PR
gcc/101643, and the test-case contains an xfail for this, which catches only:
...
(gdb) print bad^M
$2 = (0 => 0 <repeats 25 times>)^M
...

Fix this by updating the xfail pattern.

Tested on x86_64-linux.