Add locks around accesses/modifications to global encodings table

This fixes segfaults and errors of the type "Encoding not found" when
using encoding-related methods and internal encoding c functions across
ractors.

Example of a possible segfault in release mode or assertion error in debug mode:

```ruby
rs = []
100.times do
  rs << Ractor.new do
    "abc".force_encoding(Encoding.list.shuffle.first)
  end
end
while rs.any?
  r, obj = Ractor.select(*rs)
  rs.delete(r)
end
```

1 files changed, 18 insertions, 0 deletions

diff --git a/test/ruby/test_encoding.rb b/test/ruby/test_encoding.rb
index 0ab357f53a..ae4e4a7cf7 100644
--- a/test/ruby/test_encoding.rb
+++ b/test/ruby/test_encoding.rb
@@ -136,4 +136,22 @@ class TestEncoding < Test::Unit::TestCase
       assert "[Bug #19562]"
     end;
   end
+
+  def test_ractor_force_encoding_parallel
+    assert_ractor("#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      $-w = nil
+      rs = []
+      100.times do
+        rs << Ractor.new do
+          "abc".force_encoding(Encoding.list.shuffle.first)
+        end
+      end
+      while rs.any?
+        r, _obj = Ractor.select(*rs)
+        rs.delete(r)
+      end
+      assert rs.empty?
+    end;
+  end
 end