lib: Update samd21 code with local changes

A minor change is necessary to work with GCC's -flto and to work with
common bootloaders.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

lib/samd21/samd21a/gcc/gcc/startup_samd21.c

@@ -105,7 +105,7 @@ void PTC_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler
 void I2S_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
 
 /* Exception Table */
-__attribute__ ((section(".vectors")))
+__attribute__ ((section(".vectors"))) __attribute__((externally_visible))
 const DeviceVectors exception_table = {
 
         /* Configure Initial Stack Pointer, using linker-generated symbols */