There is a scenario where a user does not specify -fprofile-instr-generate on compilation command but on the linker command only for various reasons. In such cases, since no object is instrumented, the llvm_prof_xxx sections won't exist and linker won't generate __start__.. and _stop__.. symbols leading to linker time unsat.
Making start_../stop_.. weak extern symbols solve the problem when building executable, but PC relative dynamic reloc is not allowed so shared library case will still fail.
THis simple patch fixes the problem. It won't have any effect on regular use cases.
It has another added benefit -- it allows us to conditionally turn off generation of name sections completely (depending on user option) to reduce binary/profile data size -- that feature will come later.