Add signal handling to the register object.

- Add the StepIfSignalHandler function to the Regs object that checks
  if the code is in a signal handler.
- Add tests for new code, also add a test that unwinds through a signal
  handler.
- Slight modification to Elf to fail if a bad machine type is encountered.
  Add tests for this.

Bug: 23762183

Test: Ran unit tests.
Change-Id: Idafa1105d00b91a9343d7464ac9ed1cb95830963
diff --git a/libunwindstack/tests/RegsFake.h b/libunwindstack/tests/RegsFake.h
index e796c9b..f998da7 100644
--- a/libunwindstack/tests/RegsFake.h
+++ b/libunwindstack/tests/RegsFake.h
@@ -33,6 +33,7 @@
 
   uint64_t GetAdjustedPc(uint64_t, Elf*) override { return 0; }
   void SetFromRaw() override {}
+  bool StepIfSignalHandler(Memory*) override { return false; }
   bool GetReturnAddressFromDefault(Memory*, uint64_t*) { return false; }
 };