The default install path only checked whether bun existed, so an
x86_64 bun running under Rosetta installed an x86_64 omp on Apple
Silicon (AVX warning, ~11s startup). Compare bun's process.arch to
the host, detecting Apple Silicon via `sysctl -in hw.optional.arm64`
so Rosetta can't spoof it, and fall back to the prebuilt native
binary on mismatch; `--source` now errors actionably. install_binary
also derives the arch from the real host instead of the
Rosetta-translated `uname -m`.
Fixes#6268
roboomp
329db086f4