[Build, K/N] Remove registration and usages of convention.plugins.platformInfo

The entire concept of conventions is deprecated in Gradle. Here it was making the build logic implicit and hard for understanding
^KTI-1473 In Progress

kotlin-native/backend.native/build.gradle

@@ -3,6 +3,7 @@
  * that can be found in the LICENSE file.
  */
 
+import org.jetbrains.kotlin.PlatformInfo
 import org.jetbrains.kotlin.cpp.CppConsumerPlugin
 import org.jetbrains.kotlin.cpp.CppUsage
 import org.jetbrains.kotlin.cpp.DependencyHandlerExKt
@@ -78,7 +79,7 @@ kotlinNativeInterop {
         dependsOn ":kotlin-native:libllvmext:${NativePluginKt.lib("llvmext")}"
         defFile 'llvm.def'
         compilerOpts "-I$llvmDir/include", "-I${rootProject.project(':kotlin-native:llvmDebugInfoC').projectDir}/src/main/include", "-I${rootProject.project(':kotlin-native:libllvmext').projectDir}/src/main/include"
-        if (isMac()) {
+        if (PlatformInfo.isMac()) {
             // $llvmDir/lib contains libc++.1.dylib too, and it seems to be preferred by the linker
             // over the sysroot-provided one.
             // As a result, libllvmstubs.dylib gets linked with $llvmDir/lib/libc++.1.dylib.