Enable AArch64 processor feature detection unconditionally (#4034)

std::arch::is_aarch64_feature_detected!() is now part of stable
Rust, so we can always use it.

Copyright (c) 2022, Arm Limited.

.github/workflows/main.yml

@@ -62,7 +62,7 @@ jobs:
         submodules: true
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2021-12-15
+        toolchain: nightly-2022-04-27
 
     # Build C API documentation
     - run: sudo apt-get update -y && sudo apt-get install -y libclang1-9 libclang-cpp9
@@ -178,7 +178,7 @@ jobs:
     # flags to rustc.
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2021-12-15
+        toolchain: nightly-2022-04-27
     - run: cargo install cargo-fuzz --vers "^0.11"
     # Install OCaml packages necessary for 'differential_spec' fuzz target.
     - run: sudo apt install -y ocaml-nox ocamlbuild ocaml-findlib libzarith-ocaml-dev