Allow compiling on aarch64-apple-darwin

[maxfierke]

This a follow-up to #98 and adds the actual support for compiling an omnibus build of Crystal for aarch64-apple-darwin.

My prior belief was that LLVM 11 was needed, but it seems only a recent XCode (12.2+, I think) with the right linker is needed and LLVM 10 seems to work fine.

The LLVM changes to add the AArch64 target will require someone on core to update the x86_64 LLVM 10 build and add the arm64 build in S3, so I might need some assistance with that (hacked around it locally with source path: to test my local builds). I'm happy to supply a build for arm64 if no one on core has access to an M1 machine.

While I'm assuming official builds will require further work on getting Circle CI, etc. setup with M1 Macs, this at least clears the way for unofficial omnibus builds by community members (such as myself) which could also be used to seed the initial ARM64 builds for Homebrew as well.

PR in draft until #98 or equivalent is merged and LLVM builds are available to update llvm_bin definition

[maxfierke]

would probably recommend #104 over this, but if universal builds are a no-go, I may return to this and expand to allow at least for cross-compilation from x86_64 machine (e.g. CI) and not require being on Apple Silicon

[maxfierke]

Closing this out in favor of #104