Include the currently active GC in RUBY_DESCRIPTION

This will add +MOD_GC to the version string and Ruby description when
Ruby is compiled with shared gc support.

When shared GC support is compiled in and a GC module has been loaded
using RUBY_GC_LIBRARY, the version string will include the name of
the currently active GC as reported by the rb_gc_active_gc_name function
in the form

+MOD_GC[gc_name]

[Feature #20794]

Author: eightbitraptor

command_line/dash_v_spec.rb

@@ -9,6 +9,7 @@
       ruby_exe(nil, args: '-v').sub("+PRISM ", "").should include(RUBY_DESCRIPTION.sub("+PRISM ", ""))
     end unless (defined?(RubyVM::YJIT) && RubyVM::YJIT.enabled?) ||
                (defined?(RubyVM::RJIT) && RubyVM::RJIT.enabled?) ||
+               (ENV['RUBY_GC_LIBRARY'] && ENV['RUBY_GC_LIBRARY'].length > 0) ||
                (ENV['RUBY_MN_THREADS'] == '1')
   end
 end

command_line/rubyopt_spec.rb

@@ -25,12 +25,12 @@
   guard -> { RbConfig::CONFIG["CROSS_COMPILING"] != "yes" } do
     it "prints the version number for '-v'" do
       ENV["RUBYOPT"] = '-v'
-      ruby_exe("").sub("+PRISM ", "")[/\A.*/].should == RUBY_DESCRIPTION.sub("+PRISM ", "")
+      ruby_exe("").sub("+PRISM ", "").sub(/\+GC(\[\w+\]\s|\s)?/, "")[/\A.*/].should == RUBY_DESCRIPTION.sub("+PRISM ", "").sub(/\+GC(\[\w+\]\s|\s)?/, "")
     end
 
     it "ignores whitespace around the option" do
       ENV["RUBYOPT"] = ' -v '
-      ruby_exe("").sub("+PRISM ", "")[/\A.*/].should == RUBY_DESCRIPTION.sub("+PRISM ", "")
+      ruby_exe("").sub("+PRISM ", "").sub(/\+GC(\[\w+\]\s|\s)?/, "")[/\A.*/].should == RUBY_DESCRIPTION.sub("+PRISM ", "").sub(/\+GC(\[\w+\]\s|\s)?/, "")
     end
   end