* config.gcc: Merge kfreebsd*-gnu with linux* and add knetbsd*-gnu.
* config/i386/linux.h: Allow overriding of LINK_EMULATION,
  DYNAMIC_LINKER and register names in sc_ structure.
* config/kfreebsd-gnu.h: New. kfreebsd-gnu followup for linux.h.
* config/i386/kfreebsd-gnu.h: New. Ditto for i386-kfreebsd-gnu.
* config/knetbsd-gnu.h: New. Ditto for knetbsd-gnu.
* config/i386/knetbsd-gnu.h: New. Ditto for i386-knetbsd-gnu.
* config/kfreebsdgnu.h: Remove.
* config/t-kfreebsd-gnu: Likewise.
* config/i386/kfreebsdgnu.h: Likewise.

From-SVN: r83577