[OpenMP] Prefault all synchornous copies on APUs.

On APUs, align memory size after which we prefault to the same value as the size after which we switch from asynchronous to synchronous memory copies.
Prefault memory copy size can still be controlled with independent environment variable for performance profiling.

Change-Id: I1549aeb4c5fadcfdf03490a115e230106e2b4381

Author: carlobertolli

openmp/libomptarget/plugins-nextgen/amdgpu/src/rtl.cpp

@@ -2480,7 +2480,7 @@ struct AMDGPUDeviceTy : public GenericDeviceTy, AMDGenericDeviceTy {
         OMPX_SyncCopyBack("LIBOMPTARGET_SYNC_COPY_BACK", true),
         OMPX_APUPrefaultMemcopy("LIBOMPTARGET_APU_PREFAULT_MEMCOPY", "true"),
         OMPX_APUPrefaultMemcopySize("LIBOMPTARGET_APU_PREFAULT_MEMCOPY_SIZE",
-                                    2 * 1024 * 1024), // 2MB
+                                    1 * 1024 * 1024), // 1MB
         AMDGPUStreamManager(*this, Agent), AMDGPUEventManager(*this),
         AMDGPUSignalManager(*this), Agent(Agent), HostDevice(HostDevice) {}