8338930: StringConcatFactory hardCoded string concatenation strategy

Reviewed-by: redestad, liach

src/java.base/share/classes/java/lang/System.java

@@ -2647,6 +2647,10 @@ public final class System {
                 return String.COMPACT_STRINGS ? String.LATIN1 : String.UTF16;
             }
 
+            public byte stringCoder(String str) {
+                return str.coder();
+            }
+
             public int getCharsLatin1(long i, int index, byte[] buf) {
                 return StringLatin1.getChars(i, index, buf);
             }