target/riscv: Replace MO_TE by mo_endian (MIPS extension)

Replace compile-time MO_TE evaluation by runtime mo_endian()
one, which expand target endianness from DisasContext.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260318103122.97244-12-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>