Add AArch64 x18 virtualization for Linux guests

[Sangho Lee (sangho2)]

This PR adds support for AArch64 x18 virtualization for Linux guests. The x18 register is a general-purpose register on Linux whereas a reserved platform register on macOS and Windows. To work around this mismatch, this PR lets the rewriter replaces Linux guests' x18 access with memory-slot-backed access. Confirm this works using Linux-on-Linux as a test platform. It has some known coverage limitations like indirect branch with x18.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/enum_variant_added.ron

Failed in:
  variant GateMetadata:X18 in /home/runner/work/litebox/litebox/litebox_syscall_rewriter/src/aarch64.rs:248
  variant GateMetadata:X18CompareBranch in /home/runner/work/litebox/litebox/litebox_syscall_rewriter/src/aarch64.rs:253
  variant GateMetadata:X18Adr in /home/runner/work/litebox/litebox/litebox_syscall_rewriter/src/aarch64.rs:258