ref: 1eadcba462d82b73eecfcc0d7aabd56cdc3d13e1 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET