Enable RTOS with GCC_CR

1. Provide the required __end__ symbol
2. Call software_init_hook() if present, which in turn starts the RTOS

The fix was applied for all targets with a TOOLCHAIN_GCC_CR folder,
but it only works with LPC1768 and LPC4088 because of incomplete or
missing support for GCC_CR and/or the RTOS for the other targets.
Tested by running RTOS_1, RTOS_2 and RTOS_3 with LPC1768 and LPC4088.