Build: add kotlin.build.useIRForLibraries to enable JVM IR for libraries separately

The next step for JVM IR adoption in our project is going to be enabling
`kotlin.build.useIR`, but keeping `kotlin.build.useIRForLibraries`
disabled until we fix all remaining ABI incompatibility issues for
library code.

libraries/stdlib/jdk8/build.gradle

@@ -84,6 +84,8 @@ compileTestKotlin {
     ]
 }
 
+configureJvmIrBackend(project)
+
 compileJava9Sources(project, 'kotlin.stdlib.jdk8')
 
 test {