[PowerPC][docs] Update IBM official docs in Compiler Writers Info page

Summary:
Just realized that most of the links in this page are deprecated.
So update some important reference here:
* adding PowerISA 3.0B/2.7B
* adding P8/P9 User Manual
* ELFv2 ABI and errata

Move deprecated ones into "Other documents..".

Reviewers: #powerpc, hfinkel, nemanjai

Reviewed By: hfinkel

Subscribers: shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68817

llvm-svn: 374428