LVBS: Revise VTL switch loop and add run_thread support

[sangho2]

This PR revises VTL switch loop including VTL0 state saving/restoring, and
adds run_thread support.

This PR uses assembly code to carefully switch between VTLs while
saving/restoring VTL0 general-purpose registers and extended states.

Also, it adds run_thread support for the LVBS platform and revises the syscall
handling routine based on that of the Linux userland platform.

[sangho2]

.

[wdcui]

Overall, the code looks good to me. I left some comments below.

[wdcui]

Thank you for making efforts to improve this PR!

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/inherent_method_missing.ron

Failed in:
  PerCpuVariables::kernel_stack_top, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/9734d6c8b5dbe5cc67645953a2d69b856ea9665c/litebox_platform_lvbs/src/host/per_cpu_variables.rs:55

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/struct_missing.ron

Failed in:
  struct litebox_platform_lvbs::syscall_entry::SyscallContextRaw, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/9734d6c8b5dbe5cc67645953a2d69b856ea9665c/litebox_platform_lvbs/src/syscall_entry.rs:54

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field vtl1_state of struct PerCpuVariables, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/9734d6c8b5dbe5cc67645953a2d69b856ea9665c/litebox_platform_lvbs/src/host/per_cpu_variables.rs:38
  field cr2 of struct VtlState, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/9734d6c8b5dbe5cc67645953a2d69b856ea9665c/litebox_platform_lvbs/src/mshv/vtl_switch.rs:46

--- warning repr_c_plain_struct_fields_reordered: struct fields reordered in repr(C) struct ---

Description:
A public repr(C) struct had its fields reordered. This can change the struct's memory layout, possibly breaking FFI use cases that depend on field position and order.
        ref: https://doc.rust-lang.org/reference/type-layout.html#reprc-structs
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/repr_c_plain_struct_fields_reordered.ron

Failed in:
  VtlState.rax moved from position 3 to 2, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:52
  VtlState.rbx moved from position 4 to 3, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:53
  VtlState.rcx moved from position 5 to 4, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:54
  VtlState.rdx moved from position 6 to 5, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:55
  VtlState.rsi moved from position 7 to 6, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:56
  VtlState.rdi moved from position 8 to 7, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:57
  VtlState.r8 moved from position 9 to 8, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:58
  VtlState.r9 moved from position 10 to 9, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:59
  VtlState.r10 moved from position 11 to 10, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:60
  VtlState.r11 moved from position 12 to 11, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:61
  VtlState.r12 moved from position 13 to 12, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:62
  VtlState.r13 moved from position 14 to 13, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:63
  VtlState.r14 moved from position 15 to 14, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:64
  VtlState.r15 moved from position 16 to 15, in /home/runner/work/litebox/litebox/litebox_platform_lvbs/src/mshv/vtl_switch.rs:65