libunwindstack: add Regs::Clone.

Add a way to copy a Regs object.

Bug: http://b/77296294
Test: libunwindstack_test
Change-Id: I28aff510aa3e1b677d5ae46dc3bfe7652817ce52
diff --git a/libunwindstack/tests/RegsFake.h b/libunwindstack/tests/RegsFake.h
index ede16b3..d6ca9b7 100644
--- a/libunwindstack/tests/RegsFake.h
+++ b/libunwindstack/tests/RegsFake.h
@@ -58,6 +58,8 @@
   void FakeSetReturnAddress(uint64_t return_address) { fake_return_address_ = return_address; }
   void FakeSetReturnAddressValid(bool valid) { fake_return_address_valid_ = valid; }
 
+  Regs* Clone() override { return nullptr; }
+
  private:
   ArchEnum fake_arch_ = ARCH_UNKNOWN;
   uint64_t fake_pc_ = 0;
@@ -83,6 +85,8 @@
   bool SetPcFromReturnAddress(Memory*) override { return false; }
   bool StepIfSignalHandler(uint64_t, Elf*, Memory*) override { return false; }
 
+  Regs* Clone() override { return nullptr; }
+
  private:
   uint64_t fake_pc_ = 0;
   uint64_t fake_sp_ = 0;