[PGO] Ensure profile section symbols are created (linux)

- This is to handle a corner case where profile lib is linked
  in but non of the modules are instrumented (On linux, since
  we avoided the overhead to emit runtime hook use functions so
  this is the side effect of that size optimization).
- Added a profile runtime test case to cover all scenarios of
  shared library builds. 

Differential Revision: http://reviews.llvm.org/D14468

llvm-svn: 253098