Avoid longjmp()-catching compilation errors in cross-ports.

Avoid longjmp()-catching compilation errors in cross-ports.

* doc/gdbint.texinfo:  Update GET_LONGJMP_TARGET, L_SET doc.
* irix4-nat.c, mips-nat.c (JB_ELEMENT_SIZE, get_longjmp_target):
Move from mips-tdep.c and tm-{irix3,mips}.h.
* mips-nat.c:  Remove a bunch of code that was ifdef'd out of
native MIPS ports.
* nm-irix3.h, nm-mips.h (GET_LONGJMP_TARGET):  Move from tm-irix3.h
and tm-mips.h.

* ultra3-nat.c (register_addr):  Move from ultra3-xdep.c.
(fetch_core_registers):  Fix bfd_seek arguments.