LPC810 has only 4KB of flash, thus avoiding dead code is really nice.
Here the NVIC interrupt setup was pulled from the us_ticker code even
if no code is using timer events.

This also adds ARM_GCC support for TARGET_LPC81X.

LPC81X and LPC82X support Cortex M0+ VTOR register, so it is not
necessary to put non-init text at 0x200.