util: Use SSE intrinsics in _mesa_lroundeven{f,}.

util: Use SSE intrinsics in _mesa_lroundeven{f,}.

gcc actually generates this for us now that we use -fno-math-errno
(which is weird, since lrintf()/lrint() don't set errno) but clang still
does not. Presumably helps MSVC as well.

Reduced .text size by 8.5k with gcc before -fno-math-errno.

   text     data      bss      dec      hex  filename
4935850   195136    26192  5157178   4eb13a  i965_dri.so before
4927225   195128    26192  5148545   4e8f81  i965_dri.so after

Reviewed-by: Roland Scheidegger <sroland@vmware.com>