investment in unix x86_64 path parity

[brandonros]

addresses parts of #160

Summary

This brings the Unix x86_64 paths much closer to parity by wiring up the missing loader, syscall, and API-interception pieces for:

• macOS x86_64 Mach-O binaries
• Linux x86_64 ELF binaries

It also hardens the existing macOS AArch64 hello coverage and adds focused fixtures/tests for the new x86_64 paths.

What changed

Shared API ABI handling

• Added a small shared ABI helper for Unix API shims
• Updated the Linux libc and macOS libSystem gateways to read arguments / write return values for both:
  • AArch64
  • x86_64 SysV

macOS x86_64 support

• Added x86_64 Mach-O detection and loader front-door support
• Added macOS x86_64 initialization
• Routed x86_64 syscall through a macOS syscall gateway instead of falling into the Windows path
• Added x86_64 Unix API interception in set_rip()
• Reused the existing libSystem/chained-fixup machinery for x86_64 Mach-O loading

Linux x86_64 support

• Added Linux stub-lib loading from maps/maps_linux
• Added a stub libc.so.6
• Extended ELF dynamic-link handling to use:
  • DT_NEEDED
  • DT_RELA
  • DT_JMPREL
• Implemented relocation patching for:
  • R_X86_64_GLOB_DAT
  • R_X86_64_JUMP_SLOT
• Added symbol/address lookup maps for loaded ELF stub libraries
• Added Linux x86_64 Unix API interception in set_rip()
• Added a minimal __libc_start_main startup bridge for the dynamic hello-world path

Tests

• Hardened the existing macOS AArch64 hello/libc test so it now asserts success instead of logging and passing
• Added macOS x86_64 hello tests for:
  • raw syscall flow
  • libc/API interception flow
• Added a Linux x86_64 dynamic ELF hello test covering:
  • _start
  • __libc_start_main
  • main
  • puts
  • exit

Tooling / repo support

• Added ignore rules for local Zig/Cargo cache artifacts used while building fixtures

Validation

Ran:

• cargo check -p libmwemu --target x86_64-apple-darwin
• cargo test -p libmwemu macho64 --target x86_64-apple-darwin -- --nocapture
• cargo test -p libmwemu elf64lin_dynamic_hello --target x86_64-apple-darwin -- --nocapture

Notes

• This PR focuses on hello-world-level runtime bring-up and interception coverage.
• Many libc/libSystem helpers are still stubbed or todo!() and remain follow-up work.
• There are a few adjacent formatting-only changes in touched files; the functional intent of this PR is the Unix x86_64 bring-up described above.