scons: Fix Mac OS X SCons build on 32-bit CPUs.

scons: Fix Mac OS X SCons build on 32-bit CPUs.

The Mac OS X SCons build failed on 32-bit CPUs starting with commit
2f6d47a7c8d6e69e5154de44115aab9ba35a41d2 during linking of graw-null.
The build succeeds though on a 64-bit CPU. See FDO bug 29117.

This was the compiler error.
scons: building associated VariantDir targets: build/darwin-x86-debug
  Linking build/darwin-x86-debug/gallium/targets/graw-null/libgraw.dylib ...
Undefined symbols:
  "_lp_swizzled_cbuf", referenced from:
      _lp_swizzled_cbuf$non_lazy_ptr in libllvmpipe.a(lp_rast.os)
      _lp_swizzled_cbuf$non_lazy_ptr in libllvmpipe.a(lp_rast_tri.os)
     (maybe you meant: _lp_swizzled_cbuf$non_lazy_ptr)
  "_lp_dummy_tile", referenced from:
      _lp_dummy_tile$non_lazy_ptr in libllvmpipe.a(lp_rast.os)
      _lp_dummy_tile$non_lazy_ptr in libllvmpipe.a(lp_rast_tri.os)
      _lp_dummy_tile$non_lazy_ptr in libllvmpipe.a(lp_setup.os)
     (maybe you meant: _lp_dummy_tile$non_lazy_ptr)

The patch adds -fno-common to all Mac OS X builds to work around this issue.