tests: posix: common: userspace: temporarily disable qemu_arc_em

This build would fail in CI for reasons that are not obvious.

```
west twister -i -p qemu_arc/qemu_arc_em \
  -s tests/posix/common/portability.posix.common.userspace

scripts/build/gen_kobject_placeholders.py did not reserve \
  enough space for kobject rodata.
```

It's the only platform that fails this way.

Exclude it from this test temporarily until #82059 is fixed.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>