mesa: Drop mmx optimizations on Haiku

* Prevents compatibility problems. As Haiku
  doesn't use rtasm anymore, it's kind of
  pointless.
diff --git a/scons/gallium.py b/scons/gallium.py
index 98671f7..7207c6b 100755
--- a/scons/gallium.py
+++ b/scons/gallium.py
@@ -361,8 +361,7 @@
                 ccflags += [
                     '-mstackrealign', # ensure stack is aligned
                     '-march=i586', # Haiku target is Pentium
-                    '-mtune=i686', # use i686 where we can
-                    '-mmmx' # use mmx math where we can
+                    '-mtune=i686' # use i686 where we can
                 ]
         if env['machine'] == 'x86_64':
             ccflags += ['-m64']