[K/N] Convert :kotlin-native:backend.native buildscript to kts

Use folder structure from the rest of the project and build it by
the bootstrap compiler.

settings.gradle

@@ -882,6 +882,7 @@ if (buildProperties.isKotlinNativeEnabled) {
     include ':kotlin-native:endorsedLibraries:kotlinx.cli'
     include ':kotlin-native:Interop:StubGenerator'
     include ':kotlin-native:backend.native'
+    project(':kotlin-native:backend.native').projectDir = "$rootDir/kotlin-native/backend.native/compiler/ir/backend.native" as File
     include ':kotlin-native:Interop:Runtime'
     include ':kotlin-native:Interop:Indexer'
     include ':kotlin-native:Interop:JsRuntime'
@@ -897,6 +898,7 @@ if (buildProperties.isKotlinNativeEnabled) {
     include ':kotlin-native:libclangext'
     include ':kotlin-native:llvmInterop'
     include ':kotlin-native:backend.native:tests'
+    project(':kotlin-native:backend.native:tests').projectDir = "$rootDir/kotlin-native/backend.native/tests" as File
     include ':kotlin-native:prepare:kotlin-native-compiler-embeddable'
     include ':native:kotlin-test-native-xctest'
     include ':native:cli-native'