Nuke the hilariously out of date suggestion to unpack llvm-gcc 4.2 as
part of getting started with LLVM.

The LLVM getting started document is in woeful need of attention. I may
get to some of this, but some random notes for folks interested:

1) We need to separate the getting started steps for folks who are
   interested in the core LLVM libs and nothing else, folks interested
   in a nifty C++ toolchain and nothing else, and folks interested in
   both.
2) We should include documentation for both release archives, svn, and
   git in equal portion, and we should document all of the various
   repositories of interest: llvm, clang, clang-tools-extra,
   compiler-rt, lld, libcxx, test-suite.
3) We should document the CMake build. We should probably document the
   CMake build first, and give a fall-back set of docs for the Makefile
   build for the use cases where that is still the preferred solution.
   This would more closely match the use cases that folks in the open
   source community are likely to have, and would remove a point of
   discrepancy between Linux, Windows, and Mac instructions.
4) Probably a ton of other modernization stuff that I've not thought of
   here.

Anyways, if anyone at all is interested, please help clean up this
document. It is much needed.