build: add target native SDK as downloadable dependency

also improve Gradle native toolchains configuration

buildSrc/src/main/groovy/org/jetbrains/kotlin/CompileToBitcode.groovy

@@ -73,6 +73,8 @@ class CompileCppToBitcode extends DefaultTask {
             executable "$project.llvmDir/bin/clang++"
             args '-std=c++11'
 
+            args "--sysroot=$project.sysrootDir"
+
             args compilerArgs
 
             args "-I$headersDir"

buildSrc/src/main/groovy/org/jetbrains/kotlin/NativeInteropPlugin.groovy

@@ -174,6 +174,10 @@ class NamedNativeInteropConfig implements Named {
                     args "-linker:" + linker
                 }
 
+                String sysrootArg = "--sysroot=$project.sysrootDir"
+                compilerOpts += sysrootArg
+                linkerOpts   += sysrootArg
+
                 args compilerOpts.collect { "-copt:$it" }
                 args linkerOpts.collect { "-lopt:$it" }