Pull GetSoftwareBreakpointPCOffset into base class

Pull GetSoftwareBreakpointPCOffset into base class

Summary:
This function encodes the knowledge of whether the PC points to the
breakpoint instruction of the one following it after the breakpoint is
"hit". This behavior mainly(*) depends on the architecture and not on the
OS, so it makes sense for it to be implemented in the base class, where
it can be shared between different implementations (Linux and NetBSD
atm).

(*) It is possible for an OS to expose a different API, perhaps by doing
some fixups in the kernel. In this case, the implementation can override
this function to implement custom behavior.

Reviewers: krytarowski, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D52532

llvm-svn: 343409