It seems newer MacOS kernels are sending mach exceptions for watchpoints
which contain the hardware register number as opposed to the address
which triggered the exception. Also, ARM64 seems to have switched to
sending _EXC_I386_SGL as medata[0] for this exception type.
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