target/hppa: Speed up hppa_is_pa20()

target/hppa: Speed up hppa_is_pa20()

Although the hppa_is_pa20() helper is costly due to string comparisons
in object_dynamic_cast(), it is called quite often during memory lookups
and at each start of a block of instruction translations.
Speed hppa_is_pa20() up by calling object_dynamic_cast() only once at
CPU creation and store the result in the is_pa20 of struct CPUArchState.

Signed-off-by: Helge Deller <deller@gmx.de>
Co-developed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20241231190620.24442-7-philmd@linaro.org>