cpymem for RISC-V with v extension

cpymem for RISC-V with v extension

gcc/
	* config/riscv/riscv-protos.h (riscv_vector::expand_block_move):
	Declare.
	* config/riscv/riscv-v.cc (riscv_vector::expand_block_move):
	New function.
	* config/riscv/riscv.md (cpymemsi): Use riscv_vector::expand_block_move.
	Change to ..
	(cpymem<P:mode>) .. this.

gcc/testsuite/
	* gcc.target/riscv/rvv/base/cpymem-1.c: New test.
	* gcc.target/riscv/rvv/base/cpymem-2.c: Likewise.

Co-Authored-By: Juzhe-Zhong <juzhe.zhong@rivai.ai>