[PPC64] Helper for offset from a function's global entry to local entry. [NFC]

The PPC64 elf V2 abi defines 2 entry points for a function. There are a few
places we need to calculate the offset from the global entry to the local entry
and how this is done is not straight forward. This patch adds a helper function
mostly for documentation purposes, explaining how the 2 entry points differ and
why we choose one over the other, as well as documenting how the offsets are
encoded into a functions st_other field.

Differential Revision: https://reviews.llvm.org/D52231

llvm-svn: 342603