Implement basic loader for Linux aarch64

[jeffbailey]

Summary

This implements a basic arm64 loader for Linux, and all the currently
enabled linker tests pass. TLS is not implemented, and functions
using it will have undefined behaviour. Notably, the TLS test is
currently disabled on x86_64.

Much of the structure is copied from x86_64 to allow for a refactoring
of the start code between architectures.

Tested:
ninja libc_loader_tests on aarch64-linux.

Co-authored-by: Raman Tenneti <[e-mail]>

[jeffbailey]

Fix pasto in header.

[jeffbailey]

Sorry for the bad push, still learning arcanist =(

This should restore the whole patch.

[tschuett]

You are mixing aarch64 and ARM64. I always associate ARM64 with Apple and aarch64 with the rest (Unix).

[jeffbailey]

In D119641#3317112, @tschuett wrote:

You are mixing aarch64 and ARM64. I always associate ARM64 with Apple and aarch64 with the rest (Unix).

I haven't observed that difference (like in https://www.kernel.org/doc/html/latest/arm64/index.html, Debian architecture name), but I'm happy to shift if that's what's preferred.

[tschuett]

Interesting. I didn't know about it.

[sivachandra]

Thanks for working on this.

libc/loader/linux/aarch64/start.cpp

-//===-- Implementation of crt for x86_64 ----------------------------------===//
+//===-- Implementation of crt for aarch64 ---------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "config/linux/app.h"