adding mips/mipsel support for libffi2

[Matthias Klose <doko@cs.tu-berlin.de>]

gcc-3.4 has support for mips, but not yet for mipsel. Any volunteers
to add this for this architecture? Looks like _MIPS_SIM isn't defined
on this platform.

gcc-3.3 doesn't define _MIPS_SIM, neither on mips nor mipsel. A patch
from you mips guys would be welcome.

Thanks, Matthias

PS: ffitarget.h in libffi2-dev is in the wrong directory, so please
    fix before experimenting.