In particular:
Add timerfd constants and functions, from sys/timerfd.h.
Add EFD_SEMAPHORE and group all EFD_ constants together.
Add sigtimedwait function, from signal.h.
Add missing fallocate constants and functions, from linux/falloc.h and fcntl.h.
Add xattr functions, from sys/xattr.h.
Add SCHED_ and SEEK_ constants, from linux/sched.h and bits/seek_constants.h.
Add rlimit functions, from sys/resource.h.
Add RENAME_ constants, from stdio.h.
Add ino64_t type, from sys/types.h.

Make semver check workable

add wexecv, wexecve, wexecvp, wexecvpe

Add `MSG_` constants for Xtensa.

FreeBSD: machine register structs

Add constants and eventfd to illumos

linux hexagon: add missing defines

This commit defines ETIMEDOUT, SIGSTKSZ, MINSIGSTKSZ

Use ctest2 to drop old dependencies

Add `ip_mreqn` on musl targets

FreeBSD: Follow upstream `WIFSIGNALED` change

Change branch specification

remove SmartOS constants not applicable to illumos or Solaris

SmartOS is a downstream distribution of illumos, and carries a number of
additional patches that are not commonly available in other illumos
distributions.  They are also not found in Oracle Solaris.  With these
removed, the libc tests pass on a stock illumos system running
OpenIndiana.

Bump version to 0.2.71

Add `llvm-tools` component

Import `core::ops` to use `Range` syntax

This is required on rust-lang/rust.

Add types for newlib on Xtensa.

Add `TCP_KEEP{INTVL,CNT}` to macOS

linux: ptrace: Add definition for PTRACE_SYSEMU/_SINGLESTEP