No longer needed by the looks of it!

The libc crate is used as a dependency of the Rust compiler. Its build
system passes `--cfg dox` to all crates when generating their
documentation. libc's documentation is generated when the build system
is asked to generate the compiler documentation because `cargo doc`
automatically documents all dependencies.

When the dox configuration option is enabled, libc disables its
dependency on the core crate and provides the necessary definitions
itself. The dox configuration option is meant for generating
documentation for a multitude of targets even if the core crate for that
target is not installed. However, when documenting the compiler, it's
not necessary to do that; we can just use core or std as usual.

This change is motivated by the changes made to the compiler in
rust-lang/rust#48171. With these changes, it's necessary to provide
implementations of the Clone and Copy traits for some primitive types in
the library that defines these traits (previously, these implementations
were provided by the compiler). Normally, these traits (and thus the
implementations) are provided by core, so any crate that uses
`#![no_core]` must now provide its own copy of the implementations.

Because libc doesn't provide its own copy of the implementations yet,
and because the compiler's build system passes `--cfg dox` to libc,
generating the documentation for the compiler fails when generating
documentation for libc. By renaming the configuration option, libc will
use core or std and will thus have the necessary definitions for the
documentation to be generated successfully.

Add sparc64-unknown-linux-gnu and x86_64-sun-solaris.
Remove aarch64-unknown-linux-musl.

CloudABI is a sandboxed UNIX-like runtime environment, based on the
principle of capability-based security. As CloudABI is intended to be
cross-platform, the system call layer is specified here:

    https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt

From these definitions, we automatically generate C and Rust bindings.
The latter is published on crates.io:

    https://crates.io/crates/cloudabi

My goal is to implement libstd for CloudABI in such a way that it uses
the C library as little as possible; only in places where it would ease
interfacing with C code (e.g., thread creation). In places where
constants in the C library are directly based on the CloudABI
specification (e.g., errnos), use the constants provided by the cloudabi
crate.

In preparation for eventually having a non-Emscripten based wasm32 target, this
commit makes `libc` the crate an empty library on wasm32 targets that are not
with `target_os = "emscripten"`. This may eventually get filled out over time,
but for now it's all empty!

The wcstombs() function conforms to ISO/IEC 9899:1999 (``ISO C99'')

In preparation for rust-lang/rust#37545 this is adding the appropriate
directives to libc to get included.

strnlen is used to find the length of a C string that may be
lacking a terminal NUL character. Whilst it is possible to
implement the equivalent functionality in rust code, it is
cleaner, simpler and removes the need for duplication of tricky
functionality to get right. It also makes it easier to port
C code snippets to rust.

Note that strnlen is not part of POSIX or C99.