x86_64: Fix SSE4.2 libmvec atan2 function accuracy [BZ #28765]

This patch fixes SSE4.2 libmvec atan2 function accuracy for following
inputs to less than 4 ulps.

{0x1.bcab29da0e947p-54,0x1.bc41f4d2294b8p-54}   4.19888 ulps
{0x1.b836ed678be29p-588,0x1.b7be6f5a03a8cp-588} 4.09889 ulps

This fixes BZ #28765.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>