Add support for .gnu.attributes sections

[amshafer]

This adds partial support for SHT_GNU_ATTRIBUTES sections, which is of primary importance on powerpc systems since the endianness is specified as a file tag. For example see the Tag_GNU_Power_ABI_FP tag.

The details on the section format can be found here in section 4.3.6: https://static1.squarespace.com/static/59c4375b8a02c798d1cce06f/t/59d55a5a32601e2413a23988/1507154526861/ELF+for+ARM.pdf

This series only handles file tags, since they are the most straightforward. The symbol and section tag format requires being able to interact with ULEB128-encoded values, which I did not find an easy way to do without bringing in lots of third party dependencies. The file tags are the most common however, and paired with #508 allows for copying ppc64 shared libraries.

[philipc]

Thanks for the PR.

ULEB128 support is fairly simple. We could add it to Bytes instead of needing to add dependencies. That can be left for a later PR though.

[philipc]

Do you know how to create a file with a .gnu.attributes section? I tried compiling with powerpc-linux-gnu-gcc and powerpc64-linux-gnu-gcc but it doesn't produce this section.

[amshafer]

I was seeing it with little endian, so maybe powerpc64le-linux-gnu-gcc is the target you need?