test: move some tests into ARM directory

Rather than requiring ARM support for the ELF tests (which is odd), move the
tests that require ARM into a subdirectory to use lit to disable them if the
support is not present.  Play this game to prevent disabling the ELF tests on
the Windows build bots as they have caught issues in the past with interactions
between various platforms.

llvm-svn: 210408

llvm/test/MC/ELF/ARM/lit.local.cfg

@@ -0,0 +1,4 @@
+targets = set(config.root.targets_to_build.split())
+if not 'ARM' in targets:
+  config.unsupported = True
+