Use older JDK toolchains for compiling stdlib and stdlib-jdk7

To prevent using newer JDK API in kotlin sources of these projects.
Note that java sources (of stdlib mainly) are still compiled with JDK 8.

#KT-51907

libraries/stdlib/jdk7/build.gradle

@@ -57,6 +57,8 @@ artifacts {
 }
 
 compileKotlin {
+    kotlinJavaToolchain.toolchain.use(JvmToolchain.getToolchainLauncherFor(project, JdkMajorVersion.JDK_1_7))
+    kotlinOptions.jvmTarget = "1.8"
     kotlinOptions.freeCompilerArgs = [
             "-Xallow-kotlin-package",
             "-Xmultifile-parts-inherit",

libraries/stdlib/jvm/build.gradle

@@ -105,7 +105,9 @@ DexMethodCountKt.dexMethodCount(project) { task ->
 }
 
 compileKotlin {
+    kotlinJavaToolchain.toolchain.use(JvmToolchain.getToolchainLauncherFor(project, JdkMajorVersion.JDK_1_6))
     kotlinOptions {
+        jvmTarget = "1.8"
         freeCompilerArgs = [
                 "-Xallow-kotlin-package",
                 "-Xmultifile-parts-inherit",