[docs] Delete tons of bad information in the requirements section of the
getting started guide.

Some highlights:
- I heard there was this Clang compiler that you could use for your
  host compiler. Not sure though.
- We no longer have a GCC frontend with weird build restrictions.
- Windows is doing a bit better than partially supported.
- We nuked everything to do with itanium.
- SPUs? Really?
- Xcode 2.5 and gcc 4.0.1 are really not a concern -- they don't work.
- OMG, we actually tried building LLVM on Alpha? Really?
- PowerPC works pretty well these days.

There is still a lot of stuff here I'm pretty dubious about, but I nuked
most of what was actively misleading, out of date, or patently wrong.
Some of it (mingw stuff especially) isn't really lacking, its just that
the comments here were actively wrong. Hopefully folks that know those
platforms can add back correct / modern information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202370 91177308-0d34-0410-b5e6-96231b3b80d8