Link docs for other platforms in docs.rs

Verify that only non-technical breaking changes are applied to libc

Closes #270 .

cc @alexcrichton so this would be a solution to #270 that uses rust-semverver to check that the API of `libc` contains only non-technical breaking changes.

This is a WIP and uses a fork of `rust-semverver` for now, but I've sent PRs upstream already. This is the only idea I have for solving #270 . `rust-semverver` is not perfect, but it can deal with functions, consts, and simple structs just fine, and that's pretty much everything that libc uses.

cc @ibabushkin

Some other notes:

* we have to compile `rust-semverver` for each toolchain version, and it depends on `cargo` so we have to build ~160 dependencies. Using `cache: cargo` breaks everything.

Add arm-uclibc definitions

I used `buildroot-2017.05` to build armv7 cross compiler, `uclibc-1.0.24` and `libc-test`. I've executed `libc-test` on armv7-based SoC, using modified `libc-test` to account for the environment. Modified libc-test is not included in this PR because it is too dependent on cross compilation and execution environment.

Fix typo in `sem_t` alignment for mips32-uclibc

Closes #270 .

cleanup libc-test for OpenBSD

here a cleanup for libc-test for OpenBSD

Some elements (compat for old and now unsupported OpenBSD versions) could be removed, but I think it is better to address them after this PR is merged.

the testsuite for OpenBSD still pass with it (well, with #1280)

r? @gnzlbg

openbsd: glob_t recently changed to conform posix (use size_t)

on openbsd, upcoming stable version have changed glob_t definition to follow posix definition (using `size_t` instead of `int` on some fields)

the change preserves bitrig definition

r? @gnzlbg

Clean up Redox, CloudABI, DragonflyBSD, and NetBSD in libc-test/build.rs

Implement more extra_traits

Finishing the implementation of rust-lang/rust#57715 now that all platforms are tested in CI. I plan to expand this CI to all platforms that need this treatment (solarish, empscripten, freebsd, and netbsd), but thought I'd get the part I've already started running in CI since I can't test this changes locally.

Cleanup Windows libc-test build.rs

Newlib fixes.