Refactor stdsimd

This commit:

* renames `coresimd` to `core_arch` and `stdsimd` to `std_detect`

* `std_detect` does no longer depend on `core_arch` - it is a freestanding
  `no_std` library that only depends on `core` - it is renamed to `std_detect`

* moves the top-level coresimd and stdsimd directories into the appropriate
  crates/... directories - this simplifies creating crate.io releases of these crates

* moves the top-level `coresimd` and `stdsimd` sub-directories into their
  corresponding crates in `crates/{core_arch, std_detect}`.

library/stdarch/crates/std_detect/src/lib.rs

@@ -0,0 +1,37 @@
+//! Run-time feature detection for the Rust standard library.
+//!
+//! To detect whether a feature is enabled in the system running the binary
+//! use one of the appropriate macro for the target:
+//!
+//! * `x86` and `x86_64`: [`is_x86_feature_detected`]
+//! * `arm`: [`is_arm_feature_detected`]
+//! * `aarch64`: [`is_aarch64_feature_detected`]
+//! * `mips`: [`is_mips_feature_detected`]
+//! * `mips64`: [`is_mips64_feature_detected`]
+//! * `powerpc`: [`is_powerpc_feature_detected`]
+//! * `powerpc64`: [`is_powerpc64_feature_detected`]
+
+#![unstable(feature = "stdsimd", issue = "27731")]
+#![feature(const_fn, integer_atomics, staged_api, stdsimd)]
+#![feature(doc_cfg, allow_internal_unstable)]
+#![cfg_attr(feature = "cargo-clippy", allow(clippy::shadow_reuse))]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    deny(clippy::missing_inline_in_public_items,)
+)]
+#![cfg_attr(target_os = "linux", feature(linkage))]
+#![cfg_attr(all(target_os = "freebsd", target_arch = "aarch64"), feature(asm))]
+#![no_std]
+
+#[cfg(test)]
+#[macro_use(println)]
+extern crate std;
+
+extern crate libc;
+
+#[macro_use]
+extern crate cfg_if;
+
+#[doc(hidden)]
+#[unstable(feature = "stdsimd", issue = "27731")]
+pub mod detect;