Adds the low-level support for watchpoints (aka data breakpoints) to
the native linux/amd64 backend.
Does not add user interface or functioning support for watchpoints
on stack variables.
Updates #279
As proc/native is arch related, it should move some functions to arch-relate file. And this patch can help us to separate the architecture code, make code tidy. So that the merge of arm64 code later will not cause chaos.(#118)
