fix: enable monotonic clock on llvm backend

[waterlens]

cc @Young-Flash

[peter-jerry-ye-code-review]

Consider documenting target platform requirements

Category
Maintainability
Code Snippet
"monotonic_clock_native.mbt": ["native", "llvm"]
Recommendation
Add a comment or documentation explaining why both 'native' and 'llvm' targets are needed and what functionality each provides
Reasoning
Future maintainers may not understand why both targets are necessary. Having clear documentation about the purpose of each target platform helps with long-term maintenance and prevents accidental removal of required targets.