target/ppc: Fix warning with clang-15

target/ppc: Fix warning with clang-15

When compiling for windows-arm64 using clang-15, it reports a sometimes
uninitialized variable. This seems to be a false positive, as a default
case guards switch expressions, preventing to return an uninitialized
value, but clang seems unhappy with assert(0) definition.

Change code to g_assert_not_reached() fix the warning.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230221153006.20300-5-pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>