ref: f3b5bcffceb05ce2b9f8c19d2e0721f3e54098e8 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET