ref: f6a6ccb1d56c0f6cf3677b6d7f31da0c8c362510 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET