Pull FixupBreakpointPCAsNeeded into base class

Summary:
This function existed (with identical code) in both NativeProcessLinux
and NativeProcessNetBSD, and it is likely that it would be useful to any
future implementation of NativeProcessProtocol.

Therefore I move it to the base class.

Reviewers: krytarowski

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D52719

llvm-svn: 343683