Single `ret` on qemu-virt — the CPU is already at qemu's configured
rate. Body is target-conditional via .ifdef TARGET_QEMU_VIRT; the C6
arm contains a .error so it fails loudly the first time someone
builds for the C6 without writing the PLL sequence.
Per @verify: kat-only, the test asserts shape only — function exists,
body is exactly one instruction (ret), no callee-saved writes.
