Initial version of AArch64 support.

[akosthekiss]

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 rust-lang/compiler-rt#5 .

[rust-highfive]

r? @pcwalton

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[richo]

Thanks for this- I'm working on a power port and this is a pretty good todo list for things I have left!

[eddyb]

Can't the 32-bit and 64-bit architectures reuse the same tests, one for each?

[akosthekiss]

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)?

[alexcrichton]

@akiss77 would you mind rolling this into #19790? (same comment here)

[akosthekiss]

Rolled, so closing this PR as well.