Build: Switch ARMClang minsizerel to Oz

As it was in old buildsystem, to keep code size as small as possible.

Change-Id: I7b9db151bb38ca0121a28b76ab62ad2346a93eda
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index 003fe99..ae36579 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -134,6 +134,8 @@
     if (CMAKE_GENERATOR STREQUAL "Ninja")
         set( CMAKE_DEPFILE_FLAGS_ASM "--depend=<OBJECT>.d")
     endif()
+
+    set(CMAKE_C_FLAGS_MINSIZEREL "-Oz -DNDEBUG")
 endmacro()
 
 # Configure environment for the compiler setup run by cmake at the first