[rubygems/rubygems] Drop support for IRIX

The IRIX OS is no longer maintained with the last release being 16 years
ago.

https://github.com/rubygems/rubygems/commit/5381c6a871

lib/bundler/vendor/thor/lib/thor/shell/basic.rb

@@ -425,7 +425,7 @@ class Bundler::Thor
       end
 
       def unix?
-        RUBY_PLATFORM =~ /(aix|darwin|linux|(net|free|open)bsd|cygwin|solaris|irix)/i
+        RUBY_PLATFORM =~ /(aix|darwin|linux|(net|free|open)bsd|cygwin|solaris)/i
       end
 
       def truncate(string, width)