single-binary: Make hw/arm/ common

Since v3
- QAPI structure renamed as QemuTargetInfo
- MachineClass::get_valid_cpu_types() runtime
- target_aarch64() checking SysEmuTarget value
- Remove CONFIG_TCG #ifdef'ry in hw/arm/

Since v2:
- More comments from Pierrick addressed
- Use GList to register valid CPUs list
- Remove all TARGET_AARCH64 uses in hw/arm/

Since v1:
- Dropped unrelated / irrelevant patches
- Addressed Pierrick comments
- Added R-b tag
- Only considering machines, not CPUs.

Hi,

At this point this series is mostly a draft for Pierrick.

After introducing the generic TargetInfo API [*], we implement
the ARM variants, then use the API to remove target-specific code,
allowing to eventually remove the target-specific arm_ss[] source
set in meson.build, having all objects in arm_common_ss[].

Regards,

Phil.

Available here, based on tcg-next:
https://gitlab.com/philmd/qemu/-/tags/single-binary-hw-arm-rfc-v4