Add fstatat64 on Linux/Android

Add support for aarch64-unknown-linux-musl

This adds support for aarch64-unknown-linux-musl as requested in rust-lang/rust#44779 by @alexcrichton

The new file `ci/docker/aarch64-unknown-linux-musl/Dockerfile` essentially merges the aarch64-unknown-linux-gnu and x86_64-unknown-linux-musl Dockerfiles.

The bigger changes are under `src/unix/notbsd/linux/`, though they're fairly superficial:

Previously, some constants could be shared between all 64-bit musl triples, but aarch64 differs, so a number of things were moved from `musl/b64/mod.rs` to specific arches: `musl/b64/powerpc64.rs` and `musl/b64/x86_64.rs`, with the aarch64-specific differences being added to `musl/b64/aarch64.rs`.

Similarly, some constants moved from `musl/mod.rs` to lower levels: `musl/b32/mod.rs`, `musl/b64/powerpc64.rs`, and `musl/b64/x86_64.rs`, with the aarch64-specific differences added to `musl/b64/aarch64.rs`.

Finally, some things that were true of all Linux builds moved from `mod.rs` into lower levels: `mips/mod.rs`, `other/mod.rs`, `musl/b32/mod.rs`, `musl/b64/powerpc64.rs`, and `musl/b64/x86_64.rs`, with the aarch64-specific differences added to `musl/b64/aarch64.rs`.

Testing:

All linux-based triples under `ci/docker` were run through `ci/run-docker.sh` successfully, which checks that the size and alignment of definitions match for each triple, among other tests.  (The local build of rust from rust-lang/rust#44779 was set for the aarch64-unknown-linux-musl build.)

I also confirmed that it has "good style!" according to `ci/style.rs`.

Signed-off-by: Tom Kirchner <tjk@amazon.com>
Signed-off-by: Ben Cressey <bcressey@amazon.com>

Haiku fixes for nix

Unfortunately, these differ on Haiku, and using #[cfg(...)] is
not allowed in unix/mod.rs.

This reverts commit f816434f.

Add utimensat for Solaris

Add clock_gettime related functions to macOS

Fixes https://github.com/rust-lang/libc/issues/776. The constants were retrieved on macOS 10.12.6. Tried adding `clock_nanosleep` too, but it doesn't seem like macOS' libc includes that yet.

Doesn't work on iOS.

Define DCCP constants

This PR defines constants needed to utilize [DCCP](https://en.wikipedia.org/wiki/Datagram_Congestion_Control_Protocol) in Linux and NetBSD.

[Redox] add net functions, fcntl, and close

This adds net functions and two file functions: fcntl and close.

It also adds related constants.

The net functions are in a separate file for better organization.

Bump to 0.2.31

Tweak x86 gnu images

Apparently the 17.10 image broke?

Apparently the 17.10 image broke?

fuchsia: mxio was renamed to fdio

As a side effect of the rename of the Magenta kernel to Zircon, the mxio library was renamed to fdio, and the linker directive needs to be updated.

Define additional IPPROTO_* constants.

Define additional IPPROTO_* constants, e.g. for DCCP and SCTP. These constants are standard across all system types -- the values are those assigned to the protocol by the IANA:
https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml

Bring back cfg_target_vendor, which is still used

r? @alexcrichton https://github.com/rust-lang/rust/pull/44515#issuecomment-330036756

Update s3 download locations

Add additional interface flags (IFF_)

Adds the three missing IFF_ constants (IFF_LOWER_UP, IFF_DORMANT, and IFF_ECHO) per nix-rust/nix#764

Make stdbuild a cargo feature

Also invert the default feature / no std arrangement. The default
still uses std, but no_std now requires opt-in instead of opt-out.

This is work toward removing the shim in rustc.