bootstrap/codegen_ssa: ship llvm-strip and use it for -Cstrip
Fixes#131206.
- Includes `llvm-strip` (a symlink to `llvm-objcopy`) in the compiler dist artifact so that it can be used for `-Cstrip` instead of the system tooling.
- Uses `llvm-strip` instead of `/usr/bin/strip` for macOS. macOS needs a specific linker and the system one is preferred, hence #130781 but that doesn't work when cross-compiling, so use the `llvm-strip` utility instead.
cc #123151
8dee3e978a