ipred_smooth

Benchmarks:
- Kendryte K230:
intra_pred_smooth_w4_8bpc_c:        392.6 ( 1.00x)
intra_pred_smooth_w4_8bpc_rvv:      311.3 ( 1.26x)
intra_pred_smooth_w8_8bpc_c:       1204.1 ( 1.00x)
intra_pred_smooth_w8_8bpc_rvv:      488.9 ( 2.46x)
intra_pred_smooth_w16_8bpc_c:      3885.9 ( 1.00x)
intra_pred_smooth_w16_8bpc_rvv:     796.6 ( 4.88x)
intra_pred_smooth_w32_8bpc_c:      9305.7 ( 1.00x)
intra_pred_smooth_w32_8bpc_rvv:    1806.7 ( 5.15x)
intra_pred_smooth_w64_8bpc_c:     23043.0 ( 1.00x)
intra_pred_smooth_w64_8bpc_rvv:    4344.3 ( 5.30x)

- spacemiT K1:
intra_pred_smooth_w4_8bpc_c:        384.1 ( 1.00x)
intra_pred_smooth_w4_8bpc_rvv:      322.2 ( 1.19x)
intra_pred_smooth_w8_8bpc_c:       1177.6 ( 1.00x)
intra_pred_smooth_w8_8bpc_rvv:      507.1 ( 2.32x)
intra_pred_smooth_w16_8bpc_c:      3801.2 ( 1.00x)
intra_pred_smooth_w16_8bpc_rvv:     814.4 ( 4.67x)
intra_pred_smooth_w32_8bpc_c:      9103.1 ( 1.00x)
intra_pred_smooth_w32_8bpc_rvv:     980.8 ( 9.28x)
intra_pred_smooth_w64_8bpc_c:     22540.1 ( 1.00x)
intra_pred_smooth_w64_8bpc_rvv:    2319.3 ( 9.72x)