Use vector unit for RISC-V

[xypron]

RISC-V hardware with vector extension is readily available.

Please, use vector intrinsics as described in https://github.com/riscv-non-isa/rvv-intrinsic-doc/blob/v0.11.x/rvv-intrinsic-api.md if __riscv_v is defined.

[lemire]

Would you be willing to prepare a pull request ?

Testing and reviewing is not a problem but someone needs to write the code.