llvmpipe: Skip false exp2 failure in lp_test_arit due to buggy MSVCRT.
llvmpipe: Skip false exp2 failure in lp_test_arit due to buggy MSVCRT.

64bits MSVCRT's exp2f(-inf) returns -inf instead of 0.  Tested with
MSVC 2013's CRT.  (I haven't tried 2015 yet.)

Also this does not happen with MinGW.

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler