kvm-bindings: add s390x KVM bindings

[vishnubijukumar]

Add generated UAPI bindings for s390x-unknown-linux-gnu, including fam-wrappers for kvm_reg_list and kvm_irq_routing, serde support for common KVM structs, and documentation updates. Fix packed __vector128 Debug and align bindgen helper types with other architectures for zerocopy/serde.

Summary of the PR

This PR adds s390x-unknown-linux-gnu support to kvm-bindings so the crate provides generated KVM UAPI bindings for s390x with the same feature coverage as the existing architecture-specific modules.

In addition to the generated bindings, it adds fam-wrappers support for kvm_reg_list and kvm_irq_routing, serde support for common KVM structs, and the required documentation and changelog updates. It also fixes packed __vector128 Debug handling and aligns the bindgen helper types with the other architectures for zerocopy/serde compatibility.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.