libunwindstack: add Regs::Clone.

Add a way to copy a Regs object.

Bug: http://b/77296294
Test: libunwindstack_test
Change-Id: I28aff510aa3e1b677d5ae46dc3bfe7652817ce52
(cherry picked from commit 2f37a15472945194fed528cb8d3104aa4865fc4c)
diff --git a/libunwindstack/RegsMips64.cpp b/libunwindstack/RegsMips64.cpp
index 236a922..0b835a1 100644
--- a/libunwindstack/RegsMips64.cpp
+++ b/libunwindstack/RegsMips64.cpp
@@ -160,4 +160,8 @@
   return true;
 }
 
+Regs* RegsMips64::Clone() {
+  return new RegsMips64(*this);
+}
+
 }  // namespace unwindstack