RISC-V: Add vectorized strcmp and strncmp.

RISC-V: Add vectorized strcmp and strncmp.

This patch adds vectorized strcmp and strncmp implementations and
tests.  Similar to strlen, expansion is still guarded by
-minline-str(n)cmp.

gcc/ChangeLog:

	PR target/112109

	* config/riscv/riscv-protos.h (expand_strcmp): Declare.
	* config/riscv/riscv-string.cc (riscv_expand_strcmp): Add
	strategy handling and delegation to scalar and vector expanders.
	(expand_strcmp): Vectorized implementation.
	* config/riscv/riscv.md: Add TARGET_VECTOR to strcmp and strncmp
	expander.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/builtin/strcmp-run.c: New test.
	* gcc.target/riscv/rvv/autovec/builtin/strcmp.c: New test.
	* gcc.target/riscv/rvv/autovec/builtin/strncmp-run.c: New test.
	* gcc.target/riscv/rvv/autovec/builtin/strncmp.c: New test.