ref: 1cff9ab4e8412d5c6b6a3cbca720fbc4bcccecd9 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET