Promote aarch64-apple-darwin to Tier 1

Per rust-lang/rfcs#3671.

README.md

@@ -212,7 +212,6 @@ degree documented below):
 - All Rust [Tier 1 targets](https://doc.rust-lang.org/rustc/platform-support.html) are supported by
   Miri. They are all checked on Miri's CI, and some (at least one per OS) are even checked on every
   Rust PR, so the shipped Miri should always work on these targets.
-- `aarch64-apple-darwin` is supported.
 - `s390x-unknown-linux-gnu` is supported as our "big-endian target of choice".
 - For every other target with OS `linux`, `macos`, or `windows`, Miri should generally work, but we
   make no promises and we don't run tests for such targets.

ci/ci.sh

@@ -137,7 +137,7 @@ case $HOST_TARGET in
     MANY_SEEDS=16 TEST_TARGET=x86_64-pc-windows-gnu run_tests
     ;;
   aarch64-apple-darwin)
-    # Host (tier 2)
+    # Host
     GC_STRESS=1 MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 CARGO_MIRI_ENV=1 run_tests
     # Extra tier 1
     MANY_SEEDS=64 TEST_TARGET=i686-pc-windows-gnu run_tests