[gdb/testsuite] Add unsupported tests in catch_ex_std.exp

If I de-install gnatbind, I run into:
...
FAIL: gdb.ada/catch_ex_std.exp: gnatbind foo
...

Fix this by marking the test unsupported instead:
...
UNSUPPORTED: gdb.ada/catch_ex_std.exp: gnatbind foo
...

Likewise for gnatlink.

Tested on x86_64-linux, with and without gnatbind/gnatlink installed.

gdb/testsuite/ChangeLog:

2020-02-13  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/catch_ex_std.exp: Indicate unsupported if gnatbind/gnatlink
	are missing.