RISC-V: Rename and unify stringop strategy handling.

RISC-V: Rename and unify stringop strategy handling.

In preparation for the vectorized strlen and strcmp support this NFC
patch unifies the stringop strategy handling a bit.  The "auto"
strategy now is a combination of scalar and vector and an expander
should try the strategies in their preferred order.

For the block_move expander this patch does just that.

gcc/ChangeLog:

	* config/riscv/riscv-opts.h (enum riscv_stringop_strategy_enum):
	Rename...
	(enum stringop_strategy_enum): ... to this.
	* config/riscv/riscv-string.cc (riscv_expand_block_move): New
	wrapper expander handling the strategies and delegation.
	(riscv_expand_block_move_scalar): Rename function and make
	static.
	(expand_block_move): Remove strategy handling.
	* config/riscv/riscv.md: Call expander wrapper.
	* config/riscv/riscv.opt: Rename.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/cpymem-strategy-1.c: Change to
	-mstringop-strategy.
	* gcc.target/riscv/rvv/base/cpymem-strategy-2.c: Ditto.
	* gcc.target/riscv/rvv/base/cpymem-strategy-3.c: Ditto.
	* gcc.target/riscv/rvv/base/cpymem-strategy-4.c: Ditto.
	* gcc.target/riscv/rvv/base/cpymem-strategy-5.c: Ditto.