[OpenMP][plugin] Improve 521.miniswp set default for

 LIBOMPTARGET_AMDGPU_GENERIC_SPMD_TEAMS_PER_CU from 0 to 6.

time        base   new   improvement
mi300x      64.4   55.3   16.5%
mi300a     118.9   96.1   23.7%
mi250      178.1  151.4   17.7%

Author: ronlieb

offload/plugins-nextgen/amdgpu/src/rtl.cpp

@@ -2906,7 +2906,7 @@ struct AMDGPUDeviceTy : public GenericDeviceTy, AMDGenericDeviceTy {
         OMPX_QueueSize("LIBOMPTARGET_AMDGPU_HSA_QUEUE_SIZE", 512),
         OMPX_DefaultTeamsPerCU("LIBOMPTARGET_AMDGPU_TEAMS_PER_CU", 4),
         OMPX_GenericSpmdTeamsPerCU(
-            "LIBOMPTARGET_AMDGPU_GENERIC_SPMD_TEAMS_PER_CU", 0),
+            "LIBOMPTARGET_AMDGPU_GENERIC_SPMD_TEAMS_PER_CU", 6),
         OMPX_BigJumpLoopTeamsPerCU(
             "LIBOMPTARGET_AMDGPU_BIG_JUMP_LOOP_TEAMS_PER_CU", 0),
         OMPX_BigJumpLoopMaxTotalTeams(