in child processes.

`make test-spec SPECOPTS="spec/ruby/command_line/rubyopt_spec.rb -T --yjit"`
fails because `ruby_exe` doesn't seem to propagate options to child processes.

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]

[Feature #17260] One-line pattern matching using tASSOC

R-assignment is rejected instead.