Work around a stepping bug in arm64 android M

Summary:
On arm64, linux<=4.4 and Android<=M there is a bug, which prevents single-stepping from working when
the system comes back from suspend, because of incorrectly initialized CPUs. This did not really
affect Android<M, because it did not use software suspend, but it is a problem for M, which uses
suspend (doze) quite extensively.  Fortunately, it seems that the first CPU is not affected by
this bug, so this commit implements a workaround by forcing the inferior to execute on the first
cpu whenever we are doing single stepping.

While inside, I have moved the implementations of Resume() and SingleStep() to the thread class
(instead of process).

Reviewers: tberghammer, ovyalov

Subscribers: aemerson, rengolin, tberghammer, danalbert, srhines, lldb-commits

Differential Revision: http://reviews.llvm.org/D17509

llvm-svn: 261636