Improve compatibility with newer GCC+Clang, resolve some warnings

kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Utils.kt

@@ -523,7 +523,7 @@ public fun CPointer<ShortVar>.toKStringFromUtf16(): String {
         chars[index] = nativeBytes[index].toChar()
         ++index
     }
-    return String(chars)
+    return chars.concatToString()
 }
 
 /**
@@ -555,7 +555,7 @@ public fun CPointer<IntVar>.toKStringFromUtf32(): String {
             chars[toIndex++] = value.toChar()
         }
     }
-    return String(chars)
+    return chars.concatToString()
 }
 
 /**

kotlin-native/Interop/Runtime/src/native/kotlin/kotlinx/cinterop/NativeMem.kt

@@ -143,7 +143,7 @@ public fun CPointer<UShortVar>.toKStringFromUtf16(): String {
         chars[index] = nativeBytes[index].toShort().toChar()
         ++index
     }
-    return String(chars)
+    return chars.concatToString()
 }
 
 public fun CPointer<ShortVar>.toKString(): String = this.toKStringFromUtf16()

kotlin-native/Interop/Runtime/src/native/kotlin/kotlinx/cinterop/ObjectiveCImpl.kt

@@ -98,7 +98,7 @@ class ObjCNotImplementedVar<T : Any?>(rawPtr: NativePtr) : CVariable(rawPtr) {
 
 var <T : Any?> ObjCNotImplementedVar<T>.value: T
     get() = TODO()
-    set(value) = TODO()
+    set(_) = TODO()
 
 typealias ObjCStringVarOf<T> = ObjCNotImplementedVar<T>
 typealias ObjCBlockVar<T> = ObjCNotImplementedVar<T>