riscv64/pal: pal_idx_finish

Benchmarks:

- Kendryte K230:
pal_idx_finish_w4_c:       122.5 ( 1.00x)
pal_idx_finish_w4_rvv:     107.2 ( 1.14x)
pal_idx_finish_w8_c:       302.8 ( 1.00x)
pal_idx_finish_w8_rvv:     197.9 ( 1.53x)
pal_idx_finish_w16_c:      868.2 ( 1.00x)
pal_idx_finish_w16_rvv:    438.5 ( 1.98x)
pal_idx_finish_w32_c:     1966.5 ( 1.00x)
pal_idx_finish_w32_rvv:    833.0 ( 2.36x)
pal_idx_finish_w64_c:     4737.5 ( 1.00x)
pal_idx_finish_w64_rvv:   1818.3 ( 2.61x)

- Banana Pi BPI-F3:
pal_idx_finish_w4_c:       122.4 ( 1.00x)
pal_idx_finish_w4_rvv:     132.0 ( 0.93x)
pal_idx_finish_w8_c:       289.4 ( 1.00x)
pal_idx_finish_w8_rvv:     195.8 ( 1.48x)
pal_idx_finish_w16_c:      788.0 ( 1.00x)
pal_idx_finish_w16_rvv:    430.6 ( 1.83x)
pal_idx_finish_w32_c:     1699.2 ( 1.00x)
pal_idx_finish_w32_rvv:    816.3 ( 2.08x)
pal_idx_finish_w64_c:     3977.7 ( 1.00x)
pal_idx_finish_w64_rvv:   1779.4 ( 2.24x)