ref: 9b401544322fd11b72a07e77b16043e905b832cd dir: /builtins_plan9_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET