Integrate Register Saving from FeatherTrace

[prototypicalpro]

FeatherTrace saves registers pushed onto the stack to a register dump and takes a stacktrace, printing this information on reboot. While a full stacktrace as found in FeatherTrace requires changing compiler flags, saving the pushed registers would only require an inline assembly interrupt handler similar to the one found here. As a result, integrating this feature into FeatherFault would be a useful way to get around being unable to MARK some areas of code while still allowing FeatherFault to be Arduino IDE compatible (unlike FeatherTrace). An external program and the elf from the current build (hopefully compiled with -g3?) would be required to translate the saved address into a line number, but this would be useful information if the line was not MARKed.

The implementation of this ISR would be based on the link above, and would rely on the exception handling behavior in the Cortex-M0+.