meson.build: Define QEMU_ARCH in config-target.h

meson.build: Define QEMU_ARCH in config-target.h

Instead of using an ifdef ladder in arch_init.c (which we then have
to manually update every time we add or remove a target
architecture), have meson.build put "#define QEMU_ARCH QEMU_ARCH_FOO"
in the config-target.h file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210730105947.28215-5-peter.maydell@linaro.org