Fix TestRegisters on remote target.

Summary:
Using spawnSubprocess instead of forkSubprocess, so that the subprocess
spawns on the target and not the host.

Requires http://reviews.llvm.org/D7660 for cleanup.

Test Plan: TestRegisters.py passing.

Reviewers: ovyalov, vharron, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7661

llvm-svn: 229357