Set newlib optimization level to -Os (#395)

After -O2 optimization was removed in
#349 it was reported that
memory usage increased a lot
#352

The proposed solution is to use -Os which is based on -O2, but takes
care of code size that is important in embedded applications.

Author: voltur01

arm-software/embedded/arm-runtimes/CMakeLists.txt

@@ -523,6 +523,7 @@ if(C_LIBRARY MATCHES "^newlib")
                 --enable-newlib-global-atexit)
         endif()
     else()
+        set(newlib_optim_flags "-Os")
         set(newlib_flags
             --enable-newlib-io-long-long
             --enable-newlib-register-fini