Migrate to `x86_64-unknown-none`

[mkroening]

• Migrate to custom targets: Use custom targets for kernel #395
• Use nightly without built-in hermitkernel targets: Update toolchain to 2022-03-27 #408
• Align to x86_64-unknown-none: Align x86_64 target to x86_64-unknown-none #411
• Migrate to llvm-target = "x86_64-unknown-none-elf": Targets: Use none-elf instead of hermit llvm-target #414
• Switch to x86_64-unknown-none

See hermit-os/hermit-rs#197.