[MTE] [llvm-readobj] Add globals section parsing to --memtag

Global variables are described in a metadata table called
SHT_AARCH64_MEMTAG_GLOBALS_DYNAMIC. It's basically a ULEB-encoded skip
list with some other fancy encoding tricks to make it smaller. You can
see the ABI at
https://github.com/ARM-software/abi-aa/blob/main/memtagabielf64/memtagabielf64.rst#83encoding-of-sht_aarch64_memtag_globals_dynamic

This extends readelf/readobj to understand these sections.

Reviewed By: pcc, MaskRay, jhenderson

Differential Revision: https://reviews.llvm.org/D145761