PGO: Change runtime prefix from pgo to profile

These functions are in the profile runtime.  PGO comes later.

Unfortunately, there's only room for 16 characters in a Darwin section,
so use __llvm_prf_ instead of __llvm_profile_ for section names.

<rdar://problem/15943240>

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204391 91177308-0d34-0410-b5e6-96231b3b80d8