Fix exception stack unwinding for ARM Cortex-M

Fix exception stack unwinding for ARM Cortex-M

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.

gdb/ChangeLog:

        * arm-tdep.c (arm_m_exception_cache): Try use correct stack
        pointer and stack frame offset when unwinding.