[gdb/testsuite] Add xfail for PR gcc/101633

[gdb/testsuite] Add xfail for PR gcc/101633

With gcc 7.5.0, I run into:
...
(gdb) print objects^M
$1 = ((tag => object, values => ()), (tag => unused))^M
(gdb) FAIL: gdb.ada/array_of_variant.exp: scenario=minimal: print entire array
...
while with gcc 8.5.0 we have:
...
(gdb) print objects^M
$1 = ((tag => object, values => (2, 2, 2, 2, 2)), (tag => unused))^M
(gdb) PASS: gdb.ada/array_of_variant.exp: scenario=minimal: print entire array
...

This is due to a gcc PR, which I've filed at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101633 "Bug 101633 - [debug]
DW_TAG_subrange_type missing DW_AT_upper_bound".

Fix by marking this and related FAILs as XFAIL.

Tested on x86_64-linux.

gdb/ChangeLog:

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

	PR testsuite/26903
	* gdb/testsuite/gdb.ada/array_of_variant.exp: Add xfails.