Segmented loads are slow, so here we use unit-strided load and narrowing shifts.

c910:
fcmul_add_c: 2179
fcmul_add_rvv_f64: 1652

c908:
fcmul_add_c: 4891.2
fcmul_add_rvv_f64: 2399.5
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>