Fix Debug mode in CMake

Debug mode built using -O3, this patch moves -O3 to a Release only
variable so that Debug builds with just -O0.

Change-Id: I1acca68514cd3682ccf98f9a1f39904f79933a2f
Signed-off-by: Jonathan Deakin <jonathan.deakin@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/10965
Benchmark: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Jakub Sujak <jakub.sujak@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 35b6ca2..96992f4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -58,6 +58,7 @@
 # Configuration
 
 set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g -gdwarf-2 -DARM_COMPUTE_ASSERTS_ENABLED")
+set(CMAKE_CXX_FLAGS_RELEASE "-O3")
 # Default to Release Build
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   set(CMAKE_BUILD_TYPE
@@ -106,8 +107,7 @@
     -Wlogical-op
     -Wnoexcept
     -Wstrict-null-sentinel
-    -Wno-misleading-indentation
-    -O3)
+    -Wno-misleading-indentation)
 
 # Disable note popups on compiler ABI changes
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")