[llvm-readelf] Support dumping the BB address map section with --bb-addr-map.

[llvm-readelf] Support dumping the BB address map section with --bb-addr-map.

This patch lets llvm-readelf dump the content of the BB address map
section in the following format:
```
Function {
  At: <address>
  BB entries [
    {
      Offset:   <offset>
      Size:     <size>
      Metadata: <metadata>
    },
    ...
  ]
}
...
```

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D95511