Avoid using memcpy for object header in ConcurrentCopying::Copy

Memcpy can do per byte copy, this occasionally caused tearing in
the lockword when read from the source resulting in incorrect
values.

Test: test-art-host CC baker

Bug: 31423258
Change-Id: I13f856835125a3aa331530fb1f16db45d1955c8a
1 file changed