[docs] Update GoldPlugin documentation

Summary:
Updated and reorganized. Made the following additions:
1) How to see if ld.gold is installed, and whether it is the current
default.
2) How to install ld.gold as the default or alternatively use
-fuse-ld=gold.
3) Move the part about installing the newly built ld-new as the default
to the prior section and how to use --enable-gold=default to do it
automatically on install.
4) Add a note about ld.bfd supporting plugins but indicate that it is
not tested by the LLVM project and gold is the recommended linker for
use with the gold plugin.

Fixes PR32760.

Reviewers: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49490

llvm-svn: 337404