Add RISC-V 64-bit support to sequence decoding optimization path

[yunfeizhou2025]

The optimization specifically targets the decompression speed

Testing:

Platform: Zhihe A210 C908 RISC-V64
Command: ./zstd -b1 -e22 silesia.tar
Data Set: silesia.tar

Compression Level	Before (MB/s)	After (MB/s)	Improvement (%)
1	240.1	247.1	2.9%
2	179.7	183.0	1.8%
3	144.3	147.2	2.0%
4	120.9	122.0	0.9%
5	129.7	130.5	0.6%
6	140.5	141.5	0.7%
7	127.6	128.8	0.9%
8	132.6	134.0	1.1%
9	121.7	122.7	0.8%
10	115.1	116.2	1.0%
11	112.1	112.9	0.7%
12	111.2	112.3	1.0%
13	113.6	114.4	0.7%
14	111.1	111.8	0.6%
15	108.7	109.6	0.8%
16	114.3	115.3	0.9%
17	110.6	111.1	0.5%
18	104.9	105.6	0.7%
19	104.2	105.1	0.9%
20	117.2	118.1	0.8%
21	117.6	118.2	0.5%
22	117.4	118.9	1.3%