[CMake] Introduce LLVM_BUILD_EXTERNAL_COMPILER_RT option

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202363 91177308-0d34-0410-b5e6-96231b3b80d8
Stop test/CodeGen/X86/v4i32load-crash.ll targeting non-X86-64 targets.

Summary:
Fixes an issue where a test attempts to use -mcpu=x86-64 on non-X86-64 targets.
This triggers an assertion in the MIPS backend since it doesn't know what ABI to
use by default for unrecognized processors.

CC: llvm-commits, rafael

Differential Revision: http://llvm-reviews.chandlerc.com/D2877

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202369 91177308-0d34-0410-b5e6-96231b3b80d8
[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most of what was actively misleading, out of date, or patently wrong.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
[docs] Switch to the incredibly simpler "simple table" form. It now
actually looks like the table on the webpage and is entertainingly
smaller, easier to read, and easier to edit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202371 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Switch this table to the simple form as well. No content changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202372 91177308-0d34-0410-b5e6-96231b3b80d8