Do not pass -mmmx or -msse2 to the compiler

x86 GCC does not need those parameters to compile MMX or SSE assembly
or built-in intrinsics (contrary to ARM GCC w.r.t. NEON). They only
allow the compiler to issue MMX or SSE instructions for the plain C
code. We already rely on this tolerant compiler semantic for the CPU
detection code, and for some optionally accelerated filters (e.g.
deinterlace).

Disabling MMX and SSE for non-assembly code should have no or
negligible effects on the affected plugins. On the other hand, it
ensures that the plugin descriptor can be run by non-MMX/non-SSE CPUs.