coredump: Enable thread understanding for arm cortex m

Add support in arm_cortex_m python script to read thread
registers off of a thread's stack when switching context.

When CONFIG_ARM_STORE_EXC_RETURN is enabled, check the exc_return value
in thread's arch struct to determine accurately where the stack pointer
is. Also, set r7 (the frame pointer register) in case the frame pointer
is not omitted.

Only 8 registers are read from the top of the stack
for other threads present in the dump. So update the
script to reflect that.

Signed-off-by: Mark Holden <mholden@meta.com>