tcg: Add temp_readonly

In most, but not all, places that we check for TEMP_FIXED,
we are really testing that we do not modify the temporary.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>