[NativeProcessLinux] Fix a bug in instruction-stepping over thread creation

Summary:
There was a bug in NativeProcessLinux, where doing an instruction-level single-step over the
thread-creation syscall resulted in loss of control over the inferior. This happened because
after the inferior entered the thread-creation maintenance stop, we unconditionally performed a
PTRACE_CONT, even though the original intention was to do a PTRACE_SINGLESTEP. This is fixed by
storing the original state of the thread before the stop (stepping or running) and then
performing the appropriate action when resuming.

I also get rid of the callback in the ThreadContext structure, which stored the lambda used to
resume the thread, but which was not used consistently.

A test verifying the correctness of the new behavior is included.

Reviewers: ovyalov, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12104

llvm-svn: 245545