fix(jit): Remove DSB SY barriers, keep MAP_JIT + pthread_jit_write_protect_np

Address PR feedback:
- Remove DSB SY/ISB SY barriers (icache invalidation is the real issue, not barriers)
- Fix syntax error (missing let in pattern match)
- Clean up unused imports
- Apply rustfmt formatting

The fix now focuses solely on:
- MAP_JIT flag for mmap allocation on ARM64 macOS
- pthread_jit_write_protect_np(1) to switch to execute mode
- Custom Drop using munmap for MAP_JIT memory

Author: darmie