Override hard fault handlers

[jamesadevine]

The default handlers are pretty useless, and mostly just spin wait.

These are defined in the device startup script (for mbed):

https://github.com/lancaster-university/mbed-classic/blob/genuino/targets/cmsis/TARGET_Atmel/TARGET_SAM_CortexM0P/TARGET_SAMD21/TARGET_GENUINO_G18A/TOOLCHAIN_GCC_ARM/startup_samd21.c#L142

Should be pretty trivial to override as they are weak symbols.