system: Make qemu_arch_available() common code

Remove the need of per-target QEMU_ARCH. Define the
QEMU_ARCH_* constants based on SYS_EMU_TARGET_* ones,
themselves already exposed via target_arch(), allowing
to check the current target is included in @arch_bitmask.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20260213175032.32121-5-philmd@linaro.org>