rb_gv_get implicitly defines global variables

[nirvdrum]

The memcached gem calls rb_gv_get on with a variable name before calling rb_define_readonly_variable. Our implementation of rb_gv_get implicitly declares that global variable, causing the subsequent rb_define_readonly_variable call to fail:

<internal:core> core/truffle/kernel_operations.rb:34:in `define_hooked_variable_with_is_defined': Global variable $$swig_runtime_data_type_pointer4 is already defined (ArgumentError)
	from <internal:core> core/truffle/kernel_operations.rb:34:in `define_hooked_variable'
	from /Users/nirvdrum/dev/workspaces/truffleruby-ws/graal/sdk/mxbuild/darwin-aarch64/GRAALVM_A33290E019_JAVA11/graalvm-a33290e019-java11-23.0.0-dev/Contents/Home/languages/ruby/lib/truffle/truffle/cext.rb:1836:in `rb_define_hooked_variable'
	from globals.c:32:in `rb_define_readonly_variable'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/bundler/gems/memcached-bc704e00ad20/ext/rlibmemcached/rlibmemcached_wrap.c:1772:in `Init_rlibmemcached'
	from <internal:core> core/kernel.rb:226:in `gem_original_require'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/bundler/gems/memcached-bc704e00ad20/lib/memcached.rb:19:in `<top (required)>'
	from <internal:core> core/kernel.rb:234:in `gem_original_require'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/gems/bundler-2.3.22/lib/bundler/runtime.rb:60:in `block (2 levels) in require'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/gems/bundler-2.3.22/lib/bundler/runtime.rb:55:in `each'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/gems/bundler-2.3.22/lib/bundler/runtime.rb:55:in `block in require'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/gems/bundler-2.3.22/lib/bundler/runtime.rb:44:in `each'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/gems/bundler-2.3.22/lib/bundler/runtime.rb:44:in `require'
	from /Users/nirvdrum/.gem/truffleruby/3.0.3/gems/bundler-2.3.22/lib/bundler.rb:187:in `require'
	from /Users/nirvdrum/dev/workspaces/shopify_metrics/spec/dummy/config/application.rb:22:in `<top (required)>'
	from <internal:core> core/kernel.rb:293:in `require_relative'
	from /Users/nirvdrum/dev/workspaces/shopify_metrics/spec/dummy/config/environment.rb:4:in `<top (required)>'
	from <internal:core> core/kernel.rb:234:in `gem_original_require'
	from <internal:/Users/nirvdrum/dev/workspaces/truffleruby-ws/graal/sdk/mxbuild/darwin-aarch64/GRAALVM_A33290E019_JAVA11/graalvm-a33290e019-java11-23.0.0-dev/Contents/Home/languages/ruby/lib/mri/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /Users/nirvdrum/dev/workspaces/shopify_metrics/spec/spec_helper.rb:4:in `<top (required)>'
	from <internal:core> core/kernel.rb:234:in `gem_original_require'
	from <internal:/Users/nirvdrum/dev/workspaces/truffleruby-ws/graal/sdk/mxbuild/darwin-aarch64/GRAALVM_A33290E019_JAVA11/graalvm-a33290e019-java11-23.0.0-dev/Contents/Home/languages/ruby/lib/mri/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from spec/statsd_subscribers_spec.rb:3:in `<main>'

[nirvdrum]

Fixed by cb44f1b.