Irix 4.x, which stores regs a bit differently from other /proc
based systems.
* procfs.c, core-svr4.c: Move fetch_core_registers from procfs.c
to new file core-svr4.c.
* config/i386/i386sol2.mh, config/i386/i386v4.mh, config/m68k/amix.mh,
config/i386/ncr3000.mh, config/sparc/sun4sol2.mh: Add core-svr4.o
to NATDEPFILES.
* config/mips/irix4.mh: Add corelow.o to NATDEPFILES.