Demote i686-apple-darwin to Tier 2

It's deprecated by Xcode, and the version of Xcode installed by
Cirrus-CI doesn't include 32 bit libraries.

Author: asomers

CHANGELOG.md

@@ -9,6 +9,11 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 ### Changed
+
+- i686-apple-darwin has been demoted to Tier 2 support, because it's deprecated
+  by Xcode.
+  (#[1350](https://github.com/nix-rust/nix/pull/1350))
+
 ### Removed
 
 ## [0.19.1] - 28 November 2020

README.md

@@ -51,7 +51,6 @@ Tier 1:
   * aarch64-unknown-linux-gnu
   * arm-unknown-linux-gnueabi
   * armv7-unknown-linux-gnueabihf
-  * i686-apple-darwin
   * i686-unknown-freebsd
   * i686-unknown-linux-gnu
   * i686-unknown-linux-musl
@@ -74,6 +73,7 @@ Tier 2:
   * armv7-linux-androideabi
   * armv7s-apple-ios
   * i386-apple-ios
+  * i686-apple-darwin
   * i686-linux-android
   * powerpc-unknown-linux-gnu
   * s390x-unknown-linux-gnu