couple fixes for POSTMORTEM_DEBUGGING

[shadow578]

Description

This PR fixes two issues i've noticed while working on POSTMORTEM_DEBUGGING for HC32. I believe that these issues affect other ARM Cortex Mx boards as well, thus this PR.

1. In some circumstances, the UnwTabGetFunctionName would return a pointer pointing outside of the valid memory range, causing a fault when trying to print the function name in UnwReportOut. This PR adds a check to verify that the address calculated for the function name pointer is actually readable.

2. While the common fault handler (CommonHandler_C) does capture the CFSR register, the captured value is cleared when not faulted_from_exception. I cannot see why this would be needed or useful, as all it does is hide details on the cause of the fault (e.g. divide by zero, ...).

Requirements

• any ARM Cortex M Board
• POSTMORTEM_DEBUGGING enabled

Benefits

1. fixes a potential crash inside the fault handler
2. adds additional information to fault logs emmitted by POSTMOTEM_DEBUGGING

Configurations

nothing special needed in configs

[thinkyhead]

Thanks for the patch! It's hard to find complete examples of the "right" way to do this kind of thing.