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 <[email protected]>

Author: cfriedt

tests/posix/common/testcase.yaml

@@ -75,6 +75,9 @@ tests:
       - CONFIG_THREAD_STACK_INFO=n
   portability.posix.common.userspace:
     filter: CONFIG_ARCH_HAS_USERSPACE
+    # Temporarily disabled due to #82059
+    platform_exclude:
+      - qemu_arc/qemu_arc_em
     tags:
       - userspace
     extra_configs: