llvmbox

• fixes missing atomic_* implementations for darwin targets in librt
• fixes incorrect headers in sysroots caused by a bug in dedup-target-files
• includes new tool bin/llvmbox-dedup-target-files (previously only used internally)

Adds cross compilation for the following targets:

• aarch64-linux
• aarch64-macos
• aarch64-macos.11
• aarch64-macos.12
• aarch64-macos.13
• arm-linux
• i386-linux
• riscv32-linux
• riscv64-linux
• x86_64-linux
• x86_64-macos
• x86_64-macos.10
• x86_64-macos.11
• x86_64-macos.12
• x86_64-macos.13

This includes clang wrappers named with a target suffix, e.g. clang-x86_64-macos.11 builds C programs for macOS 11 x86_64.