ref: ca2fa9596b5d0d893be28c6e8fd8a52a89a0c3c6 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET