ref: a5a8a92adf39b1a4525d6b0ddebc75e6db03e5b3 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET