[K/N] Update minimal Xcode version

The next LLVM version will be 11.1.0, and we need to be sure that
Xcode supports bitcode of this version. So we bump minimal Xcode version
to 12.5, which should be OK, because by the time Kotlin 1.6.0 comes out,
Xcode 12.5 will be around 6 month old.

Also this commit simplifies tarball naming.

kotlin-native/README.md

@@ -27,7 +27,7 @@ More information can be found in the overviews of
 [Kotlin/Native](https://kotlinlang.org/docs/native-overview.html)
 and [Kotlin Multiplatform](https://kotlinlang.org/docs/multiplatform.html).
 
-On macOS Kotlin/Native requires Xcode 11 or newer.
+On macOS Kotlin/Native requires Xcode 12.5 or newer.
 
 ## Building from source
 
@@ -35,7 +35,7 @@ Prerequisites:
 *   configure Kotlin build as [specified in main readme](../ReadMe.md#build-environment-requirements)
 *   at the root directory of the repository,
     create `local.properties` file with `kotlin.native.enabled=true` line
-*   on macOS install Xcode 12 or newer
+*   on macOS install Xcode 12.5 or newer
 *   on Fedora 26+ ```yum install ncurses-compat-libs``` may be needed
 *   on recent Ubuntu ```apt install libncurses5``` is needed