[RFE] Rework the hw/cpu/ directory layout, add missing traits

Suggest two changes:

(a) Currently, there are several missing CPU traits (including those CPU
    flags that provide mitigation for the recent processor-related
    security flaws) as 'traits'; add them.

(b) Adjust and clean up the directory layout of the 'os-traits/hw/cpu/'.
    E.g. compare the old and the new (proposed) layouts:

    Old Layout
    ----------

        cpu/
        ├── aarch64.py
        ├── amd.py
        ├── __init__.py
        └── x86.py

    Proposed Layout
    ---------------

        cpu/
        ├── aarch64.py
        ├── amd.py
        ├── __init__.py
        └── x86/
           ├── amd.py
           ├── __init__.py
           └── intel.py