sdl_gfx: don't use --enable-mmx on x86_64

Even though the MMX instructions are available on x86_64 processors,
the MMX code in sdl_gfx is written in IA32-specific assembly code, and
therefore does not build on x86_64. It generates the following build
issues:

SDL_imageFilter.c: Assembler messages:
SDL_imageFilter.c:34: Error: `pusha' is not supported in 64-bit mode
SDL_imageFilter.c:38: Error: `popa' is not supported in 64-bit mode
SDL_imageFilter.c:77: Error: `pusha' is not supported in 64-bit mode
SDL_imageFilter.c:93: Error: `popa' is not supported in 64-bit mode
[...]

We fix this by only enabling MMX support in this package when the
processor supports MMX *and* it is a IA32 compatible processor.

Fixes

  http://autobuild.buildroot.org/results/b9efc611f5da487079b6be37bb7a41a3198d63b9/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>