Add a symbols_map tool for extracting identifiers from elf and r8 files

Add a symbols_map tool that can extract an identifiying hash from
and elf file or an r8 dictionary.  The tool writes the hash to a
textproto, and also supports a merge mode to combine textprotos into
a output file for inclusion in the build artifacts.

Bug: 218888599
Test: m dist
Test: symbols_map_test.go
Change-Id: Icd3ed6e5510e058c92d97c78759e7a4cfcdbb6ca