ffbuild: Refine MIPS handling

ffbuild: Refine MIPS handling

To enable runtime detection for MIPS, we need to refine ffbuild
part to support buildding these feature together.

Firstly, we fixed configure, let it probe native ability of toolchain
to decide wether a feature can to be enabled, also clearly marked
the conflictions between loongson2 & loongson3 and Release 6 & rest.

Secondly, we compile MMI and MSA C sources with their own flags to ensure
their flags won't pollute the whole program and generate illegal code.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Shiyou Yin <yinshiyou-hf@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>