Fix XHPROF_FLAGS_NO_BUILTINS

Summary: We were totally ignoring this flag. Add the right checks for it.

I also realized that with EnableRenameFunction off, builtins will never
be profiled because we call builtins via FCallBuiltin, which doesn't do
surprise checks. I documented this behavior in profiling.md.

Reviewed By: @ptarjan

Differential Revision: D1509142