new pipeline `BuildMacOS_arm64`
  - `vmImage: 'macOS-10.15' `

new `macos_arm64_script`
  - switch from using `make` to `ninja`
  - select xcode 12.4
  - set zig-cache env variables
  - build host-zig binary with xcode, link against llvm for x86_64 (target macos 10.15)
  - build arm64-zig binary with xcode and host-zig, link against llvm for arm64 (target macos 11.0)
  - ad-hoc codesign arm64 binary with linker
  - use host-zig for docgen
  - use host-zig for experimental std lib docs
  - sync final `release/` hierarchy with `linux_script`
  - use gnu-tar for good-practices (set owner, set sort)

enhance `CMakeLists.txt`
  - do not build `zig0` when cross-compiling
  - disable `BYPRODUCTS` directive `zig1.o` to avoid `ninja` error

see #8265