[docs] Improve HowToCrossCompilerBuiltinsOnArm

Some recent experience on llvm-dev pointed out some errors in the document:
- Assumption of ninja
- Use of --march rather than -march
- Problems with host include files when a multiarch setup was used
- Insufficient target information passed to assembler
- Instructions on using the cmake cache file BaremetalARM.cmake were
  incomplete

There was also insufficient guidance on what to do when various stages
failed due to misconfiguration or missing steps.

Summary of changes:
- Fixed problems above
- Added a troubleshooting section with common errors.
- Cleared up one "at time of writing" that is no longer a problem.

Differential Revision: https://reviews.llvm.org/D55709

llvm-svn: 349477