gdb: Implement native dumpcore function

Add new API for systems with native kernel support for dumping
a process on demand. Wire it into the gdb's gcore functionality.

gdb/ChangeLog:

       * target.h (supports_dumpcore, dumpcore): New
       function declarations.
       * target.c (supports_dumpcore, dumpcore): New
       functions.
       * target-delegates.c: Rebuild.
       * gcore.c (gcore_command): Use target_supports_dumpcore ()
       and target_dumpcore ().