ref: 5f7a6b7ea3c4ae1b51beffe3309e9b6b0491e71f dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET