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
