Move musl-compatible constants to linux/mod.rs

This is a continuation of #1452. I think CI is ready to run fully and the PR is ready to be merged if the pipelines pass based on the CI I've done using musl libc in my own projects.

Support Vxworks

Define the C interface to VxWorks

Define the C interface to vxWorks for x86, x86_64, ARM, ARM 64-bit, PowerPC, PowerPC SPE and PowerPC 64-bit.

Upgrade FreeBSD's CI image to release 11.3

skip roundtrip on few structs on OpenBSD

skip roundtrip on some structs on OpenBSD. it is mostly the same than other targets: dirent, utsname, utmp.

define KERN_TIMEOUT_STATS and incr KERN_MAXID on OpenBSD

`KERN_TIMEOUT_STATS` has been defined in [sys/sysctl.h (1.194)](https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/sys/sysctl.h?rev=1.194&content-type=text/x-cvsweb-markup)

Add IP_RECVERR (unix/linux_like/mod.rs)

Setup Azure Pipelines

cc @alexcrichton this needs enabling Azure Pipelines for this repo

The 11.3-release GCE image isn't working due to a bug in the included
py36-google-compute-engine package, but that bug is fixed in the latest
stable/11 images.

Add support for hexagon-unknown-linux-musl

Increase Redox & Relibc support

- Add a lot of constants from relibc
- Fix the timezone not found error found previously on Redox
- Wrap WIFEXITED _et al._ in an unsafe block to match the rest of the API
- Add support for the extra_traits feature and Redox
- Fmt

cc @jackpot51

Update wasmtime to the latest master.

The previous wasmtime revision is broken because one of its dependencies, `memoffset` 0.3, was yanked, which appears to make it unavailable. This was fixed by downgrading to `memoffset` 0.2.

Disable testing that thumbv6m-none-eabi builds properly