ref: 8fdf22d5fb5dca8b0acb1cbfccca1eda33b7d39f dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET