ref: 6a51a03b801b21f42e2eb3dfa02c99e96c86b10c dir: /plan9_builtins_amd64.s/
TEXT __builtin_clz(SB),1,$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET