aarch64 code improperly assumes it is running in a std environment

[jscatena88]

Compilation fails if you run:

cargo +nightly build --target aarch64-unknown-none --no-default-features --features=nightly

with

   Compiling crc32fast v1.3.2 (/Users/jscatena/Workspace/rust-crc32fast)
error[E0433]: failed to resolve: maybe a missing crate `std`?
 --> src/specialized/aarch64.rs:1:5
  |
1 | use std::arch::aarch64 as arch;
  |     ^^^ maybe a missing crate `std`?
  |
  = help: consider adding `extern crate std` to use the `std` crate

error[E0433]: failed to resolve: use of undeclared crate or module `std`
  --> src/specialized/aarch64.rs:10:12
   |
10 |         if std::arch::is_aarch64_feature_detected!("crc") {
   |            ^^^ use of undeclared crate or module `std`

warning: the feature `aarch64_target_feature` has been stable since 1.61.0 and no longer requires an attribute to enable
  --> src/lib.rs:40:22
   |
40 |     feature(stdsimd, aarch64_target_feature)
   |                      ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(stable_features)]` on by default

For more information about this error, try `rustc --explain E0433`.
warning: `crc32fast` (lib) generated 1 warning
error: could not compile `crc32fast` (lib) due to 2 previous errors; 1 warning emitted

[srijs]

Hi there! Have now gotten around to looking into this, could you give the following branch a try to see if it fixes things for you? Thanks!

https://github.com/srijs/rust-crc32fast/tree/fix/no-std-aarch64