ggml : riscv: add xtheadvector support

[xctan]

This PR builds upon #12530 to introduce k-quant support for the older RVV v0.7.1 implementation (xtheadvector).

Additionally, it updates zfh extension detection to use the built-in compiler macro, eliminating the need for an extra definition.

Evaluation

Build instruction

mkdir build
cd build
cmake .. -DGGML_RVV=1 -DGGML_XTHEADVECTOR=1 -DGGML_RV_ZFH=0
make -j$(nproc)

Verification

Test model: gemma-3-4b-it-GGUF, Q4_K_M quantization. The results of llama-perplexity are:

#12530 (rvv 1.0)	this PR (xtheadvector)
16.7120 +/- 0.16651	16.7120 +/- 0.16651

Performance

Using the same model as above on SG2042.

model	size	params	backend	threads	test	t/s	note
gemma3 4B Q4_K - Medium	2.31 GiB	3.88 B	CPU	32	pp512	15.73 ± 0.14	xtheadvector
gemma3 4B Q4_K - Medium	2.31 GiB	3.88 B	CPU	32	pp512	3.35 ± 0.00	scalar
gemma3 4B Q4_K - Medium	2.31 GiB	3.88 B	CPU	32	tg128	5.15 ± 0.00	xtheadvector
gemma3 4B Q4_K - Medium	2.31 GiB	3.88 B	CPU	32	tg128	2.44 ± 0.00	scalar