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

Add __errno_location on Redox, DragonFlyBSD, Haiku

Currently [`libstd`](https://github.com/rust-lang/rust/blob/909f5a049415a815b23502a5498de9a99bbf276b/src/libstd/sys/unix/os.rs#L29-L49) and the [`getrandom` crate](https://github.com/rust-random/getrandom/pull/54/files#diff-027a56068e2bf3d31dc4273ee6e07034R12) have to use external declarations in order to implement `errno_location` across all UNIX platforms. This change adds bindings for the remaining UNIX platforms, allowing everyone to just use normal `libc` bindings.

This also removes the need for a seperate [`errno-dragonfly`](https://crates.io/crates/errno-dragonfly) crate.

Links to the relevant header files:

- Redox: [`relibc`'s `bits/errno.h`](https://github.com/redox-os/relibc/blob/master/include/bits/errno.h) uses `__errno_location`
- Haiku: [`posix/errno.h`](https://github.com/luciang/haiku/blob/master/headers/posix/errno.h) uses `_errnop`
- DragonFlyBSD: [`sys/errno.h`](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/master/sys/sys/errno.h) uses `__error` just like FreeBSD (which makes sense)
  - Note that the `__error` function is inlined. I don't think this causes a problem.

Support pthread_{set,get}name_np for linux with glibc

Add missing syscalls for aarch64-unknown-linux-musl

The list of syscalls from [rust-lang-aarch64-musl-syscalls](../blob/master/src/unix/notbsd/linux/musl/b64/aarch64.rs) misses some of the syscalls that are defined in [musl-libc-aarch64-syscalls](https://git.musl-libc.org/cgit/musl/tree/arch/aarch64/bits/syscall.h.in).
We need this so that we get rid of hardcoding them inside [our project](https://github.com/firecracker-microvm/firecracker/blob/master/vmm/src/default_syscalls/filters.rs#L16

).

Signed-off-by: Diana Popa <dpopa@amazon.com>

Signed-off-by: Diana Popa <dpopa@amazon.com>

Expose signal value of siginfo_t

Currently exposes it for following platforms:

- Linux has it as enum _timer and it is exposed alongside sigval
- On FreeBSD like systems si_value follows after si_addr (according to headers)
- Darwin is similar to FreeBSD
- NetBSD and FreeBSD uses struct that contains pid, uid, and si_value. Exposed via it

P.s. I'd like to take a look at other platforms too, but these are what I know so far.
FreeBSD needs some testing though

cc @gnzlbg

Exposes value for most unix like platforms

Bump patch version

Remove AF_MAX, PF_MAX, NET_MAXID constants

These constants have already been deprecated for a few releases with
a deprecation notice, so they can finally be removed.

Closes rust-lang/libc#665

Reduce appveyor churn

This should reduce the Appveyorn churn for other projects a bit more.

Please let me know if there is anything else that can be done. Maybe the infra team could discuss whether to document some general guidelines for rust-lang and rust-lang-nursery projects, and write them down somewhere. I would be willing to help.

cc @RalfJung @Mark-Simulacrum

Update ctest version

The latest ctest version enabled the ABI roundtrip test by default, in which we initialize all types in Rust by default to some random bit-pattern, pass them to C, verify, modify, pass back to Rust, and verify.

This catches issues in the call ABI / calling convention.

This PR will silence those here for now.

These constants have already been deprecated for a few releases with
a deprecation notice, so they can finally be removed.

Closes rust-lang/libc#665

Fix link in README.md

Same as #1417, but actually against the right branch this time.

wasi: add c_schar definition

This fixes an issue when building https://github.com/capstone-rust/capstone-rs/ for WASI.

Otherwise, I get:
~~~
error[E0412]: cannot find type `c_schar` in module `libc`
 --> /home/user/capstone-rs/target/wasm32-wasi/debug/build/capstone-sys-1019f1f8759b0d05/out/capstone.rs:4:27
  |
4 | pub type __int8_t = libc::c_schar;
  |                           ^^^^^^^ help: a type alias with a similar name exists: `c_char`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0412`.
error: Could not compile `capstone-sys`.
~~~

The file with the error is generated with bindgen:

https://github.com/capstone-rust/capstone-rs/blob/a74126cd88d683c0c16029be361ee85c12934878/capstone-sys/pre_generated/capstone.rs#L3-L5

Remove new field from ucontext_t for compatibility with earlier glibc versions

Per discussion in #1410 with @gnzlbg, this is necessary to avoid struct size mismatches between Rust and C on systems with glibc < 2.28.