Discovered while debugging: djg/audioipc-2#52

These errno's are defined in <sys/errno.h> on DragonFlyBSD.

Signed-off-by: Levente Kurusa <lkurusa@acm.org>

Signed-off-by: Gerd Zellweger <gzellweger@vmware.com>

Closes #1060 .

No longer needed and this is done via other means in upstream rustc

Signed-off-by: Gerd Zellweger <mail@gerdzellweger.com>

This used to be defined multiple times in different (but not all)
architectures. Moving it to general linux module since it's an
architecture independent feature.

This commit prepares the `libc` crate to be included directly into the
standard library via crates.io. More details about this can be found on
rust-lang/rust#56092, but the main idea is that this crate now depends
on core/compiler-builtins explicitly (but off-by-default).

The main caveat here is that this activates `no_core` when building as
part of libstd, which means that it needs to explicitly have an `iter`
and `option` module for the expansion of `for` loops to work.

unix::uclibc::mips has both support for mips and mips64, but it's
currently only imported for mips.