target/xtensa: Remove tswap() calls in semihosting simcall() helper

target/xtensa: Remove tswap() calls in semihosting simcall() helper

In preparation of heterogeneous emulation where cores with
different endianness can run concurrently, replace the pair
of cpu_memory_rw_debug() + tswap() calls by put/get_user_u32()
ones, which still do the same under the hood, but simplify the
code maintenance (having less sites to do endianness code
conversion).

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <6e1e69d8-a9f3-4a30-83c8-84c5647578d5@linaro.org>