[gdb/testsuite] Fix gdb.fortran/ptype-on-functions.exp with gcc-4.8

When running test-case gdb.fortran/ptype-on-functions.exp with gfortran 4.8.5,
we run into:
...
(gdb) ptype some_module::get_number^M
type = integer(kind=4) (Type __class_some_module_Number)^M
(gdb) FAIL: gdb.fortran/ptype-on-functions.exp: ptype some_module::get_number
ptype some_module::set_number^M
type = void (Type __class_some_module_Number, integer(kind=4))^M
(gdb) FAIL: gdb.fortran/ptype-on-functions.exp: ptype some_module::set_number
...

The test-case pattern expects a "_t" suffix on "__class_some_module_Number".

The difference is caused by a gcc commit 073afca6884 'class.c
(gfc_build_class_symbol): Append "_t" to target class names to make the
generated type names unique' which has been present since gcc 4.9.0.

Fix the pattern by optionally matching the _t suffix.

Tested on x86_64-linux, with gfortran 4.8.5 and 7.5.0.

gdb/testsuite/ChangeLog:

2020-07-30  Tom de Vries  <tdevries@suse.de>

	* gdb.fortran/ptype-on-functions.exp: Make "_t" suffix on
	"__class_some_module_Number_t" optional.