Scalar dc calculation

Current benchmark:

- Kendryte K230:
inv_txfm_add_16x16_dct_dct_0_8bpc_c:     1729.4 ( 1.00x)
inv_txfm_add_16x16_dct_dct_0_8bpc_rvv:    153.2 (11.29x)

- spacemiT K1:
inv_txfm_add_16x16_dct_dct_0_8bpc_c:     1533.4 ( 1.00x)
inv_txfm_add_16x16_dct_dct_0_8bpc_rvv:    176.8 ( 8.67x)