tests: lib: lockfree: Exclude FVP SMP platforms for now

The lockfree test times out on FVP SMP configurations, hanging during
lockfree synchronization operations. This affects all C library variants
(minimal, picolibc, and - before being disabled - newlib).

The lockfree library uses Dmitry Vyukov's MPSC queue and SPSC ringbuffer
implementations with SEQ_CST atomics. While the implementations are
theoretically sound, they experience synchronization issues on the FVP
simulator in SMP mode that cause the test to hang.

This is distinct from the newlib spinlock validation failures and affects
the core lockfree algorithms themselves.

Adding redundant barriers i.e. `barrier_dmem_fence_full()` before
`atomic_get()` (DMB Before LDAR) apparently makes the test pass but this
masks symptoms without fixing the root cause:
- `barrier_dmem_fence_full()` compiles to `DMB SY` on ARM64
- LDAR already provides acquire semantics
- `DMB SY; LDAR` provides no additional ordering guarantees over
  `LDAR` alone

Signed-off-by: Nicolas Pitre <[email protected]>

Author: Nicolas Pitre

tests/lib/lockfree/testcase.yaml

@@ -2,5 +2,10 @@ tests:
   libraries.lockfree:
     platform_exclude:
       - m2gl025_miv # renode times out
+      # FVP SMP: test timeout in lockfree synchronization
+      - fvp_base_revc_2xaem/v8a/smp
+      - fvp_base_revc_2xaem/v8a/smp/ns
+      - fvp_base_revc_2xaem/v9a/smp
+      - fvp_base_revc_2xaem/v9a/smp/ns
     tags:
       - lockfree