[Arm] Cleanup arm_m_exception_cache

With this change, only valid contents of LR are accepted when unwinding
exception frames for m-profile targets.

If the contents of LR are anything but EXC_RETURN or FNC_RETURN, it
will cause GDB to print an error and/or abort unwinding of the frame as
it's an invalid state for the unwinder.

The FNC_RETURN pattern requires Security Extensions to be enabled.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>