Change format of -X arguments to require value after '='

Report a warning when an argument is passed in the old form (with the
whitespace)

 #KT-17264 Fixed

libraries/tools/binary-compatibility-validator/build.gradle

@@ -17,7 +17,7 @@ sourceSets {
 
 compileTestKotlin {
     kotlinOptions {
-        freeCompilerArgs = ['-Xdump-declarations-to', "${buildDir}/cases-declarations.json"]
+        freeCompilerArgs = ["-Xdump-declarations-to=${buildDir}/cases-declarations.json"]
     }
 }