Show the selector name found in the current argument registers in crash reports, helpful e.g. for crashes in objc_msgSend

This is a safe implementation with only hard wiring the necessary bits for where objc_msgSend stores the receiver objects based on the CPU architecture. From: http://sealiesoftware.com/blog/archive/2008/09/22/objc_explain_So_you_crashed_in_objc_msgSend.html

In the mid/long term, PLCrashReporter will include this directly.

In addition add `bit_archNameFromImageInfo` to get the current architecture only once