Enable HCC_OPT_FLUSH=1.

Requires appropriate HCC with this support   :
commit 38e392b517a46a09a3b1c8f388e6a0db3741c510


[ROCm/hip commit: ac634bf334]

projects/hip/src/hip_hcc.cpp

@@ -98,7 +98,7 @@ int HIP_SYNC_NULL_STREAM = 0;
 
 // HIP needs to change some behavior based on HCC_OPT_FLUSH :
 // TODO - set this to 1
-int HCC_OPT_FLUSH = 0;
+int HCC_OPT_FLUSH = 1;