stm32: Disable computed goto on constrained boards.

Saves ~1kiB.  Add comment to this effect to mpconfig.h.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/mpconfig.h

@@ -483,7 +483,8 @@
 /* Optimisations                                                             */
 
 // Whether to use computed gotos in the VM, or a switch
-// Computed gotos are roughly 10% faster, and increase VM code size by a little
+// Computed gotos are roughly 10% faster, and increase VM code size by a little,
+// e.g. ~1kiB on Cortex M4.
 // Note: enabling this will use the gcc-specific extensions of ranged designated
 // initialisers and addresses of labels, which are not part of the C99 standard.
 #ifndef MICROPY_OPT_COMPUTED_GOTO