InstrProf: Test the functions in the runtime

Check that the profile runtime works as expected.  This tests the
functions that are meant to be available to advanced users.

In particular, check that the `atexit()` hook can be disabled by
defining a custom `__llvm_profile_runtime` variable, that the libc
dependencies are optional, and that the various functions for writing
out files work for basic cases.

llvm-svn: 208460