For Cortex-M targets using floating-point, eg the Cortex-M4F, its not possible to get any call-stack backtrace if setting a breakpoint in ISR.

The exception stack unwinder for Cortex-M does not consider if floating-point registers was stacked or not,
further the Cortex-M has two stack pointers: MSP (Main Stack Pointer) and PSP (Process Stack Pointer).
This is not handled when GDB tries to backtrace in the exception stack unwinder.

This patch fixes this, and gives a correct call-stack backtrace from breakpoints set in a handler or ISR.

Best Regards,
Fredrik Hederstierna

Senior Software Developer
Verisure Innovation Centre
Malmoe Sweden