Update coverage sample documentation.

We need to invoke `llvm-profdata` tool from
the same toolchain as Clang (on macOS we use the one from Xcode).

(cherry picked from commit 91665b36af68b7fb3a56c0e7ed7ebb7d819adfb2)

kotlin-native/samples/coverage/README.md

diff --git a/kotlin-native/samples/coverage/README.md b/kotlin-native/samples/coverage/README.md
index 25155d9..f9c97dc 100644
--- a/kotlin-native/samples/coverage/README.md
+++ b/kotlin-native/samples/coverage/README.md
@@ -4,9 +4,8 @@
 Please note that this functionality will be incorporated into Gradle plugin so you won't need to do it by hand in the nearest future. 
 
 ### Prerequisites
-`createCoverageReport` task requires `llvm-profdata` and `llvm-cov` to be added to the `$PATH`. 
-They can be found in the Kotlin/Native dependencies dir. By default it should look like
-`$HOME/.konan/dependencies/clang-llvm-6.0.1-darwin-macos/bin`
-
+`createCoverageReport` task requires `llvm-profdata` and `llvm-cov` to be added to the `$PATH`.  
+In case of macOS, use tools from Xcode (`/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin`).  
+For Windows and Linux, use the ones from Kotlin/Native LLVM distribution (e.g. `$HOME/.konan/dependencies/clang-llvm-8.0.0-linux-x86-64/bin`).
 ### Usage
 Just run `createCoverageReport` task.