Read .note.gnu.build-id.

This will be used by heapprofd to allow us to correlate build ids with
memory leaks in libraries and binaries.

Test: m
Test: host libunwindstack_test
Test: run unwind_info against my phone's libc.so (32/64) and compare
      to readelf Build ID output.

Bug: 120186412

Change-Id: I3cefd6cce9a8733509bf35b7175eb0f967783477