[gdb/testsuite] Fix gnatmake_version_at_least

After de-installing gnatmake, I get:
...
Running src/gdb/testsuite/gdb.ada/rename_subscript_param.exp ...
ERROR: tcl error sourcing src/gdb/testsuite/gdb.ada/rename_subscript_param.exp.
ERROR: couldn't execute "gnatmake": no such file or directory
    while executing
"exec $gnatmake --version"
    (procedure "gnatmake_version_at_least" line 4)
...

Fix this by wrapping the exec call in a catch call.

Tested with and withouth gnatmake installed on x86_64-linux.

gdb/testsuite/ChangeLog:

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

	* lib/ada.exp (gnatmake_version_at_least): Wrap exec call in a catch
	call.