ggml: riscv: add riscv spacemit backend

[alex-spacemit]

This pull request adds spacemit backend. Specific optimizations have been made for the SpacemiT X60 CPU. The SpacemiT IME extended instructions are used to accelerate matrix calculations for Q4_0/Q4_1/Q4_K, along with general RVV optimizations.

model name      : Spacemit(R) X60
isa             : rv64imafdcv_zicbom_zicboz_zicntr_zicond_zicsr_zifencei_zihintpause_zihpm_zfh_zfhmin_zca_zcd_zba_zbb_zbc_zbs_zkt_zve32f_zve32x_zve64d_zve64f_zve64x_zvfh_zvfhmin_zvkt_sscofpmf_sstc_svinval_svnapot_svpbmt
mmu             : sv39
uarch           : spacemit,x60
mvendorid       : 0x710
marchid         : 0x8000000058000001

Q4_0

Model	Size	Params	backend	threads	test	t/s
Qwen2.5 0.5B	403.20 MiB	630.17 M	cpu	4	pp512	64.12 ± 0.26
Qwen2.5 0.5B	403.20 MiB	630.17 M	cpu	4	tg128	10.03 ± 0.01
Qwen2.5 1.5B	1011.16 MiB	1.78 B	cpu	4	pp512	24.16 ± 0.02
Qwen2.5 1.5B	1011.16 MiB	1.78 B	cpu	4	tg128	3.83 ± 0.06
Qwen2.5 3B	1.86 GiB	3.40 B	cpu	4	pp512	12.08 ± 0.02
Qwen2.5 3B	1.86 GiB	3.40 B	cpu	4	tg128	2.23 ± 0.02

more information at build-riscv64-spacemit.md

[alex-spacemit]

The file format and the limitations of the riscv zba extension have been resolved. @qnixsynapse