tools: initial implementation of WoA EH decoding

Add support to llvm-readobj to decode Windows ARM Exception Handling data.  This
uses the previously added datastructures to decode the information into a format
that can be used by tests.  This is a necessary step to add support for emitting
Windows on ARM exception handling information.

A fair amount of formatting inspiration is drawn from the Win64 EH printer as
well as the ARM EHABI printer.  This allows for a reasonably thorough look into
the encoded data.

llvm-svn: 210192