doc: Remove documentation of IRIX 6.4 and older as supported platforms.
* doc/gnulib-intro.texi (Target Platforms): Mention that IRIX <= 6.4 is
unsupported.
* doc/pastposix-functions/usleep.texi: Don't mention IRIX specific
workarounds.
* doc/posix-functions/nl_langinfo.texi: Likewise.
* doc/posix-functions/remainder.texi: Likewise.
* doc/posix-functions/towlower.texi: Likewise.
* doc/posix-functions/towupper.texi: Likewise.
* doc/posix-functions/vsnprintf.texi: Likewise.
* doc/posix-functions/wcscat.texi: Likewise.
* doc/posix-functions/wcschr.texi: Likewise.
* doc/posix-functions/wcscmp.texi: Likewise.
* doc/posix-functions/wcscpy.texi: Likewise.
* doc/posix-functions/wcscspn.texi: Likewise.
* doc/posix-functions/wcslen.texi: Likewise.
* doc/posix-functions/wcsncat.texi: Likewise.
* doc/posix-functions/wcsncmp.texi: Likewise.
* doc/posix-functions/wcsncpy.texi: Likewise.
* doc/posix-functions/wcspbrk.texi: Likewise.
* doc/posix-functions/wcsrchr.texi: Likewise.
* doc/posix-functions/wcsspn.texi: Likewise.
* doc/posix-headers/langinfo.texi: Likewise.
* doc/posix-headers/signal.texi: Likewise.
* doc/posix-headers/wchar.texi: Likewise.
* doc/posix-headers/wctype.texi: Likewise.
* doc/**/*.texi: Update.