8245094: Reduce overhead of initializing the default StringConcatFactory strategy

Reviewed-by: psandoz, jlaskey

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

@@ -2283,6 +2283,10 @@ public final class System {
                 return StringConcatHelper.lookupStatic(name, methodType);
             }
 
+            public long stringConcatInitialCoder() {
+                return StringConcatHelper.initialCoder();
+            }
+
             public Object classData(Class<?> c) {
                 return c.getClassData();
             }

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java

@@ -1724,12 +1724,7 @@ public final class StringConcatFactory {
         private static final long INITIAL_CODER;
 
         static {
-            try {
-                MethodHandle initCoder = JLA.stringConcatHelper("initialCoder", methodType(long.class));
-                INITIAL_CODER = (long) initCoder.invoke();
-            } catch (Throwable e) {
-                throw new AssertionError(e);
-            }
+            INITIAL_CODER = JLA.stringConcatInitialCoder();
 
             PREPENDERS = new ConcurrentHashMap<>();
             MIXERS = new ConcurrentHashMap<>();

src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java

@@ -333,6 +333,11 @@ public interface JavaLangAccess {
      */
     MethodHandle stringConcatHelper(String name, MethodType methodType);
 
+    /**
+     * Get the string concat initial coder
+     */
+    long stringConcatInitialCoder();
+
     /*
      * Get the class data associated with the given class.
      * @param c the class