[PATCH v4 15/18] perf clang: Declare BPF functions for BPF scripts automatically

Use Clang's OverlayFileSystem, add '-include' options to make builtin clang define BPF functions. After this patch BPF script writer needn't define BPF functions by their own. Add -DBUILTIN_CLANG_DEFAULT_INCLUDE to builtin clang so when adopting builtin clang BPF functions can be automatically

[PATCH v4 15/18] perf clang: Declare BPF functions for BPF scripts automatically

Use Clang's OverlayFileSystem, add '-include' options to make builtin clang define BPF functions. After this patch BPF script writer needn't define BPF functions by their own. Add -DBUILTIN_CLANG_DEFAULT_INCLUDE to builtin clang so when adopting builtin clang BPF functions can be automatically