Add full support for initing registers.

- Fixes a few bugs in untested functionality.
- Add tests for the way the register handling code is used.
- Fix a few tests that were not reaping child processes.

Bug: 23762183

Test: Ran unit tests on host (32 bit and 64 bit).
Test: Ran unit tests on angler (32 bit and 64 bit).
Change-Id: I573d6617b4f1561f6e8494d7213c52086d112d97
diff --git a/libunwindstack/tests/RegsFake.h b/libunwindstack/tests/RegsFake.h
index ff030c8..c57954c 100644
--- a/libunwindstack/tests/RegsFake.h
+++ b/libunwindstack/tests/RegsFake.h
@@ -30,6 +30,7 @@
 
   uint64_t GetRelPc(Elf*, const MapInfo*) override { return 0; }
   uint64_t GetAdjustedPc(uint64_t, Elf*) override { return 0; }
+  void SetFromRaw() override {}
   bool GetReturnAddressFromDefault(Memory*, uint64_t*) { return false; }
 };