Fix heap poisoning in UnsafeCASObject x86/x86-64 intrinsic.

Properly handle the case when the same object is passed to
sun.misc.Unsafe.compareAndSwapObject for the `obj` and
`newValue` arguments (named `base` and `value` in the
intrinsic implementation) and re-enable this intrinsic.

Also convert some reinterpret_casts to down_casts.

Bug: 12687968
Change-Id: I82167cfa77840ae2cdb45b9f19f5f530858fe7e8