[build] Enable LTO to further shrink ROM size

Signed-off-by: Jimmy Huang <jimmy.huang@intel.com>

Author: jimmy-huang

cmake/jerry.cmake

@@ -45,7 +45,7 @@ set(CMAKE_ARGS
   -DCMAKE_C_COMPILER_WORKS=TRUE
   -DCMAKE_SYSTEM_NAME=Zephyr
   -DENABLE_ALL_IN_ONE=OFF
-  -DENABLE_LTO=OFF
+  -DENABLE_LTO=ON
   -DEXTERNAL_COMPILE_FLAGS=${external_project_cflags}
   -DFEATURE_ERROR_MESSAGES=ON
   -DFEATURE_DEBUGGER=${DEBUGGER}