testsuite: mark jspace as fragile on i386.

This test has been flaky for some time and has been failing consistently on
i386-linux since 8e0446df landed.

See #24261

testsuite/tests/driver/j-space/all.T

-test('jspace', [extra_files(['genJspace']), req_target_smp, req_ghc_smp, js_broken(22573)], makefile_test, ['jspace'])
+test('jspace', [extra_files(['genJspace']), req_target_smp, req_ghc_smp, js_broken(22573),when(arch('i386'), fragile(24261))], makefile_test, ['jspace'])