Use the libc and elf crates instead of bindgen.

[vext01]

This make phdrs simpler and more lightweight.

For now this would only work on Linux, but it'd be easy to get working elsewhere with small patches to libc (see this issue).

Makes phdrs build so much faster.

[vext01]

We also had to define some of our own constants. Here are libc issues for those:

• libc should expose program header flag constants rust-lang/libc#1876
• libc is missing some program header type constants rust-lang/libc#1877

[ptersilie]

bors r+

[bors]

Build succeeded:

• buildbot/buildbot-build-script