Initial version of AArch64 support.

There are still some FIXMEs in the code (around thread locals and stack protection) but rustc already passes most of the tests on AArch64:
summary of 41 test runs: 8265 passed; 1 failed; 542 ignored; 0 measured.

Needs #19790 and https://github.com/rust-lang/compiler-rt/pull/5 .
It seems/seemed to me that they could. However, I didn't want/dare to go against the practice I found here: `fn test_hash_uint()` is already cloned for arm and x86 targets, that's why I added a new version for aarch64, cloning the x86_64 version. Shall the refactoring (merging the clones) be part of this patch (which will make it not purely aarch64-focused)?