cmd/ld: decode local entry offset from ppc64 symbols

cmd/ld: decode local entry offset from ppc64 symbols

ppc64 function symbols have both a global entry point and a local
entry point, where the difference is stashed in sym.other.  We'll need
this information to generate calls to ELF ABI functions.

Change-Id: Ibe343923f56801de7ebec29946c79690a9ffde57
Reviewed-on: https://go-review.googlesource.com/2002
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>