[Pixman] [PATCH 1/2] configure.ac: Use '-mloongson-mmi' for Loongson MMI.

[YunQiang Su]

Please reject ALL of @loongson.cn patch, as I know they are try to
KILL all other MIPS platform.
There are some extensions are Loongson-only (MMI here), while they
continue to try to set them as default.