ARMAsmBackend uses a factory method to generate binary file format specific

ARMAsmBackend uses a factory method to generate binary file format specific
objects.  There were a few FIXMEs in ARMAsmBackend.cpp suggesting the class
definitions should be in a separate file.  Starting with ARMAsmBackend, the
class definition has been put in a header file, and #includes reduced.  Each
sub-type of ARMAsmBackend is now in its own header file.

Derived types have been painted with a different color of bike-shed:

  s/DarwinARMAsmBackend/ARMAsmBackendDarwin/g
  s/ARMWinCOFFAsmBackend/ARMAsmBackendWinCOFF/g
  s/ELFARMAsmBackend/ARMAsmBackendELF/g

Finally, clang-format has been run across ARMAsmBackend.cpp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217866 91177308-0d34-0410-b5e6-96231b3b80d8