Add CI steps that verify baseline builds don't use CPU instructions
beyond their target. Uses QEMU user-mode emulation with restricted
CPU features — any illegal instruction causes SIGILL and fails the build.
Platforms verified:
- linux-x64-baseline (glibc): qemu-x86_64 -cpu Nehalem (catches AVX/AVX2)
- linux-x64-musl-baseline: same
- linux-aarch64 (glibc): qemu-aarch64 -cpu cortex-a35 (catches LSE/SVE)
- linux-aarch64-musl: same
Each step runs two smoke tests under QEMU:
1. bun --version (startup + linker + static init)
2. bun -e eval (JSC initialization + basic execution)
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