Increase available RAM for RTX on LPC11U24

The INITIAL_SP value used by RTX was originally at the 4k limit instead
of at the 8k limit.  The HEAP_START macro was defined to __HeapLimit
but the heap really starts at __end__.  __HeapLimit is just used for
some link time sanity checks to verify that the heap and stack can
grow a bit without colliding.

See https://mbed.org/questions/2065/RTOS-Memory-Usage-Confusion/ for Neil Thiessen's encountering of the LPC11U24 issue as well.