Modernize NativeProcessProtocol::GetSoftwareBreakpointTrapOpcode

return the opcode as a Expected<ArrayRef> instead of a
Status+pointer+size combo.

I also move the linux implementation to the base class, as the trap
opcodes are likely to be the same for all/most implementations of the
class (except the arm one, where linux chooses a different opcode than
what the arm spec recommends, which I keep linux-specific).

llvm-svn: 341487