profile: Add low level versions of profile buffer functions

On Darwin, compiler_rt uses magic linker symbols to find the profile
counters in the __DATA segment.  This is a reasonable method for
normal, hosted, userspace programs.  However programs with custom
memory layouts, such as the kernel, will need to tell compiler_rt
explicitly where to find these sections.

Patch by Lawrence D'Anna. Thanks!

llvm-svn: 223840