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)
15 files changed