Removed information about compiling the GCC front end. This will be in a

Removed information about compiling the GCC front end.  This will be in a
separate document that we will provide to people who request the source.
Updated the support architecture information to be a little more precise.
Added hyperlinks for all of the tools which are required for building LLVM.
This should make it easier for people to find and install these required
tools.
Italicized some of the "variables" that we use in place of absolute paths.
Added the --enable-spec2000 option to the quick start section.
Other minor changes/corrections/clarifications.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9006 91177308-0d34-0410-b5e6-96231b3b80d8