RISC-V: testsuite: Fix strcmp-run.c test.

RISC-V: testsuite: Fix strcmp-run.c test.

This fixes expectations in the strcmp-run test which would sometimes
fail with newlib.  The test expects libc strcmp return values and
asserts the vectorized result is similar to those.  Therefore hard-code
the expected results instead of relying on a strcmp call.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/builtin/strcmp-run.c: Adjust test
	expectation and target selector.
	* gcc.target/riscv/rvv/autovec/builtin/strlen-run.c: Adjust
	target selector.
	* gcc.target/riscv/rvv/autovec/builtin/strncmp-run.c: Ditto.