ref: d60399e18345e69419489f201f27f19f1a5616b2 dir: /plan9_builtins_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET