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/RegsMips.cpp b/libunwindstack/RegsMips.cpp
index c87e69b..2e6908c 100644
--- a/libunwindstack/RegsMips.cpp
+++ b/libunwindstack/RegsMips.cpp
@@ -173,4 +173,8 @@
   return true;
 }
 
+Regs* RegsMips::Clone() {
+  return new RegsMips(*this);
+}
+
 }  // namespace unwindstack