Host/target/native split for sun4.

Host/target/native split for sun4.

* Makefile.in (TSOBS): removed corelow.o.
* infptrace.c: included nat.h.
* nat-trash.h: temporary header file.  This should be removed once
  all hosts have the native/host/target split.
* configure.in: add a symlink from nat-trash.h to nat.h if no
  other nat file exists for this configuration.
* sparc-tdep.c: no longer include sys/ptrace.h.
* sparc-xdep.c: removed.  contents have been moved to nat-sparc.c.
* xm-sparc.h (ATTACH_DETACH, FETCH_INFERIOR_REGISTERS): moved to
  nat-sun4os4.h.
* nat-sparc.c, nat-sun4os4.h: new files for sun4 native support.
* config/sun4os4.mh (XDEPFILES): moved infptrace.o and inftarg.o
  to NATDEPFILES.  removed sparc-xdep.o.
  (NATDEPFILES, NAT_FILE): new macros for native support.