py: Don't include mp_optimise_value or opt_level() if compiler disabled.

Without the compiler enabled the mp_optimise_value is unused, and the
micropython.opt_level() function is not useful, so exclude these from the
build to save RAM and code size.

py/mpstate.h

@@ -199,7 +199,9 @@ typedef struct _mp_state_vm_t {
     mp_thread_mutex_t qstr_mutex;
     #endif
 
+    #if MICROPY_ENABLE_COMPILER
     mp_uint_t mp_optimise_value;
+    #endif
 
     // size of the emergency exception buf, if it's dynamically allocated
     #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF && MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE == 0