executor: sys/linux: Implement nested SYZOS loading

This commit introduces the `SYZOS_API_NESTED_LOAD_SYZOS` command to
enable running full SYZOS programs within a nested L2 guest, enhancing
fuzzing capabilities for nested virtualization.

Key changes include:
- Nested SYZOS Execution: The new command loads a SYZOS program into an
  L2 VM, setting up its execution environment.
- ABI Refinement: Program size is now passed via the shared `syzos_globals`
  memory region instead of registers, standardizing the ABI for L1 and L2.
- L2 State Management: Improved saving and restoring of L2 guest GPRs
  across VM-exits using inline assembly wrappers for Intel and AMD.
- Nested UEXIT Propagation: Intercepts EPT/NPT faults on the exit page to
  capture the L2 exit code from saved registers and forward it to L0 with
  an incremented nesting level.
- L2 Memory Management: Updates to L2 page table setup, including skipping
  NO_HOST_MEM regions to force exits, and a new `l2_gpa_to_pa` helper.

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