Add logic to the DNBArchImplX86_64/DNBArchImplI386::NotifyException() callback method

in order to distinguish the real single step exception from a watchpoint exception
which uses the same exc_type of EXC_BREAKPOINT and exc_code of EXC_I386_SGL.

This is done by checking the debug status register to find out whether the watchpoint
data break event has fired, and, if yes, stuff the data break address into the exception's
exc_sub_code field on the debugserver side for lldb to consume on the other end.

llvm-svn: 139274