package/spandsp: disable MMX on i686

package/spandsp: disable MMX on i686

MMX raises the following build failure on i686:

gsm0610_rpe.c: In function 'gsm0610_rpe_encoding':
gsm0610_rpe.c:132:5: error: invalid 'asm': invalid constraints for operand
     __asm__ __volatile__(
     ^~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/3e986c3109c392afe47fc98446a2563ac9776cf6
 - http://autobuild.buildroot.org/results/00ed4a4285b35d8ec0be09217e5b503e4820d971

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>