[obj2yaml, yaml2obj] Use Hex64 for BBAddressMap fields.

This patch let the yaml encoding use Hex64 values for NumBlocks, BB AddressOffset, BB Size, and BB Metadata.
Additionally, it changes the decoded values in elf2yaml to uint64_t to match DataExtractor::getULEB128 return type.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D95767