[FFmpeg-devel] Add support for loongarch.

[Shiyou Yin]

V2:
1. rebase.
2. Change Author email from yinshiyou at loongson.cn to
yinshiyou-hf at loongson.cn for 1/3 2/3.
3. Refine 2/3.

[PATCH v2 1/3] configure: Add support for loongarch.
[PATCH v2 2/3] avcodec: [loongarch] optimize get_cabac.
[PATCH v2 3/3] avcodec: [loongarch] Optimize decode_significance.