[PowerPC][docs] Update Embedded PowerPC docs in Compiler Writers Info page

Summary:
Embedded PowerPC are still actively supported, especially SPE...
So update some important references here:

* adding EREF
* adding SPE/VLE ref

Delete deprecated ones into "Other documents..".

Reviewers: #powerpc, jhibbits, hfinkel

Reviewed By: #powerpc, jhibbits

Subscribers: wuzish, merge_guards_bot, nemanjai, shchenz, steven.zhang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72008