[Backtracing] Rename _Backtracing to Runtime.

Move the backtracing code into a new Runtime module.  This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.

rdar://124913332

stdlib/public/RuntimeModule/get-cpu-context.asm

@@ -0,0 +1,4 @@
+;;; Placeholder for Windows assembly code
+;;; This uses different syntax from everywhere else :-(
+
+        end