target/xtensa: clean up unaligned access

target/xtensa: clean up unaligned access

Xtensa cores may or may not have hardware support for unaligned memory
access. Remove TARGET_ALIGNED_ONLY=y from all xtensa configurations and
pass MO_ALIGN in memory access flags for all operations that would raise
an exception.
Simplify use of gen_load_store_alignment by passing access size and
alignment requirements in single parameter.
Drop condition from xtensa_cpu_do_unaligned_access and replace it with
assertion.
Add a test.

Suggested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>