mesa: Drop mmx optimizations on Haiku

* Prevents compatibility problems. As Haiku
  doesn't use rtasm anymore, it's kind of
  pointless.

scons/gallium.py

@@ -361,8 +361,7 @@ def generate(env):
                 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']