Obsolete old MIPS SVr3 and SVr4 systems.
* NEWS: Mention obsoleted systems.
* configure.tgt: Mark mips*-*-sysv4* as obsolete.
* configure.host: Mark mips-dec-*, mips-little-*, mips-sony-*,
mips-*-sysv4*, mips-*-sysv*, mips-*-riscos* and mips-*-mach3* as
obsolete.
* config/mips/decstation.mh: Mark as obsolete.
* config/mips/littlemips.mh: Mark as obsolete.
* config/mips/nm-mips.h: Mark as obsolete.
* config/mips/xm-mips.h: Mark as obsolete.
* config/mips/news-mips.mh: Mark as obsolete.
* config/mips/nm-news-mips.h: Mark as obsolete.
* config/mips/riscos.mh: Mark as obsolete.
* config/mips/nm-riscos.h: Mark as obsolete.
* config/mips/xm-riscos.h: Mark as obsolete.
* config/mips/mipsv4.mt: Mark as obsolete.
* config/mips/tm-mipsv4.h: Mark as obsolete.
* config/mips/xm-mipsv4.h: Mark as obsolete.
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