hw/arm/v7m: Remove use of &first_cpu in machine_init()

hw/arm/v7m: Remove use of &first_cpu in machine_init()

When instanciating the machine model, the machine_init()
implementations usually create the CPUs, so have access
to its first CPU. Use that rather then the &first_cpu
global.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Samuel Tardieu <sam@rfc1149.net>
Message-id: 20250112225614.33723-4-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>