target/ppc: Check endianness at runtime in ppc_data_endian_env()

Rather a runtime endianness check via env MSR over
a build-time one.

Now CPU can change endianness at runtime.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20260202210106.93257-11-philmd@linaro.org>