Condition EH ABI on target_arch, not target_os.
More precise unwinder private data size specification.
Stop padding metadata

This is causing maybe_get_doc to fail when called at the top level of
the metadata and doesn't appear to be needed anymore.
auto merge of #11316 : vadimcn/rust/arm-ehabi, r=alexcrichton

Also, more precise unwinder private data size specification.
auto merge of #11297 : sfackler/rust/de-jank-encoder-2, r=alexcrichton

This is causing maybe_get_doc to fail when called at the top level of
the metadata and doesn't appear to be needed anymore.

A try build succeeded so this should be okay.