ref: fd8a2d81c3ddf6584d2b47f1b8efe30b66beb7b5 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET