pcm: dmix: assembly: fix the interface for safety (mmx)

pcm: dmix: assembly: fix the interface for safety (mmx)

- add mm0 in the clobber list if the compiler
  is aware of the mmx technology;
- otherwise, add the mmx aliased x87 floating point
  registers in the clobbers;
- the configure now checks if the compiler is aware of
  the MMX technology.

  The compiler assumes none of the mmx or x87 registers are used
  by the function. If it chooses to store some data in them, they
  will be overwritten by the chunk.
  Recall that any mmx instruction invalidate the whole set of
  x87 floating point registers.

  Note: currently does not impact the binary output.

Signed-off-by: Frédéric Recoules <frederic.recoules@orange.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>