[gdb/testsuite] Mark ptype_union.exp as unsupported for cc-with-gdb-index

[gdb/testsuite] Mark ptype_union.exp as unsupported for cc-with-gdb-index

When testing gdb with board cc-with-gdb-index, we run into:
...
FAIL: gdb.ada/ptype_union.exp: ptype global
FAIL: gdb.ada/ptype_union.exp: print global
...

The index is not supported for Ada (PR24713), and cc-with-gdb-index does not
add an index for Ada test-cases.  However, this test-case compiles C sources,
for which cc-with-gdb-index does add an index.  In gdb we load the executable
containing the index and set the language to Ada, resulting in gdb trying to
handle something that is not supported.

Fix the fail by marking this unsupported.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-06-21  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24518
	PR ada/24713
	* gdb.ada/ptype_union.exp: Mark as unsupported if executable contains
	index.