Implement GC ref map parsing and display in R2RDump (#21509)

Implement GC ref map parsing and display in R2RDump (#21509)

As part of my work on CPAOT implementation of GC ref map info
I have implemented decoder of the info so that it can be displayed
next to the import cell signatures.

This also uncovered one possible cause of R2RDump GcInfo-related
crashes that were observed by Andon and myself. It looks like Amy
in her initial implementation confused the various GC encodings
and used GC info to parse the import section auxiliary data which
actually contains the GC ref map info.

Thanks

Tomas