riscv: Omit optimized string routines when using KASAN

The optimized string routines are implemented in assembly, so they are
not instrumented for use with KASAN. Fall back to the C version of the
routines in order to improve KASAN coverage. This fixes the
kasan_strings() unit test.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240801033725.28816-2-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>