mesa: Add -fno-math-errno -fno-trapping-math to CXXFLAGS.

Not sure why I forgot to add them to CXXFLAGS in commit f55c408067 or
commit 875458b778. Cuts about 1k of .text.

   text     data      bss      dec      hex  filename
5806354   287816    29384  6123554   5d7022  i965_dri.so before
5805497   287744    29384  6122625   5d6c81  i965_dri.so after

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
1 file changed