test/MC: Introduce MC/MachO/ARM, and relocate relax-thumb2-branches.s into it.

FIXME: Restore more other arch-dependent MachO tests. (eg. r126401 and r133856)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145925 91177308-0d34-0410-b5e6-96231b3b80d8

test/MC/MachO/ARM/dg.exp

@@ -0,0 +1,5 @@
+load_lib llvm.exp
+
+if { [llvm_supports_target ARM] } {
+  RunLLVMTests [lsort [glob -nocomplain $srcdir/$subdir/*.{ll,c,cpp,s}]]
+}