[gdb/testsuite] Fix gdb.fortran/call-no-debug.exp with native-gdbserver

[gdb/testsuite] Fix gdb.fortran/call-no-debug.exp with native-gdbserver

When running test-case gdb.fortran/call-no-debug.exp with target board
native-gdbserver, I run into:
...
(gdb) PASS: gdb.fortran/call-no-debug.exp: print string_func_ (&'abcdefg', 3)
call (integer) string_func_ (&'abcdefg', 3)^M
$2 = 0^M
(gdb) FAIL: gdb.fortran/call-no-debug.exp: call (integer) string_func_ (&'abcdefg', 3)
...

The problem is that gdb_test is used to match inferior output.

Fix this by using gdb_test_stdio.

Tested on x86_64-linux.