The sym.Symbol 'Localentry' field is used only with cgo and/or
external linking on MachoPPC. Relocate it to sym.AuxSymbol since it is
infrequently used, so as to shrink the main Symbol struct.

Updates #26186

Change-Id: I5872aa3f059270c2a091016d235a1a732695e411
Reviewed-on: https://go-review.googlesource.com/125477Reviewed-by: Ian Lance Taylor <iant@golang.org>