target/alpha: Access CPUState::cpu_index via helper

target/alpha: Access CPUState::cpu_index via helper

CPUState::cpu_index is a target agnostic field, meant
for common code (i.e. accel/ and system/ folders).

Target specific code should use the CPUClass::get_arch_id()
helper, even if there is a 1:1 mapping.

In preparation of generic changes around CPU indexing,
introduce the whoami helper to access the generic
CPUState::cpu_index field.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250925010438.59755-1-philmd@linaro.org>