Use StringBuilder instead of String concat for startup code

Unfortunately the JDK's indyfied String concatenation
has a slight performance penalty at startup, so for
code we know will always be run at startup, let's not
pay that unnecessary price.
With this change we force javac to use the
StringBuilder strategy

core/runtime/pom.xml

@@ -296,6 +296,8 @@
                             </annotationProcessorPaths>
                             <compilerArgs>
                                 <arg>-AsplitOnConfigRootDescription=true</arg>
+                                <!-- As string concatenation has a non-zero impact on startup, let's disable it for this module -->
+                                <arg>-XDstringConcat=inline</arg>
                             </compilerArgs>
                         </configuration>
                     </execution>