Fix gdb.fortran "failed to extract expected results" errors

Fix gdb.fortran "failed to extract expected results" errors

When running the gdb.fortran tests array-slices.exp and lbound-ubound.exp,
the test suite throws several ERRORs on native-gdbserver/-m{32,64},
and native-extended-gdbsever/-m{32,64}:

[on native-extended-gdbserver/-m64]
Running /home/keiths/work/gdb/branches/testsuite-errors/linux/gdb/testsuite/../../../src/gdb/testsuite/gdb.fortran/array-slices.exp ...
ERROR: failed to extract expected results
ERROR: failed to extract expected results
Running /home/keiths/work/gdb/branches/testsuite-errors/linux/gdb/testsuite/../../../src/gdb/testsuite/gdb.fortran/lbound-ubound.exp ...
ERROR: failed to extract expected results for lbound

This occurs because the tests require inferior I/O which we do not have
access to while using these targets.

This patch skips these tests when running on non-native targets.