ref: 0ed8a3bd7b1bbe2a3857f2abcaabdc4edd2d8b05 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET