aarch64-unknown-linux-gnu support

[termoshtt]

While aarch64-unknown-linux-gnu target is still in Tier 2, ARMv8 is becoming popular ISA on several usage:

• AWS Graviton2 instance
• Raspberry Pi 4
  • related: Getting started on Raspberry Pi #191
• A64FX HPC cluster

[termoshtt]

ARM performance library could be a better backend https://developer.arm.com/tools-and-software/server-and-hpc/compile/arm-compiler-for-linux/arm-performance-libraries

[termoshtt]

Arm Performance Libraries provides optimized standard core math libraries for high-performance computing applications on Arm processors. This free version of the libraries provides optimized libraries for Arm® Neoverse™ N1-based Armv8 AArch64 implementations that are compatible with various versions of GCC. You do not require a license for this version of the libraries.

ARM performace library looks split from Arm Allinea Studio in 20.2 release (?), and can be used without license.
https://developer.arm.com/tools-and-software/server-and-hpc/downloads/arm-performance-libraries

It will works well on AWS M6g and other Graviton2 instances, which are based on Neoverse N1
https://aws.amazon.com/about-aws/whats-new/2020/06/amazon-ec2-c6g-r6g-instances-amazon-graviton2-processors-generally-available