PGO: Implement Darwin linker magic for instrumentation

Use Darwin linker magic to find bounds of instrumentation data sections
at link time instead of runtime.

<rdar://problem/15943240>

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204302 91177308-0d34-0410-b5e6-96231b3b80d8