build: make clang using gcc toolchain on Linux

also fix Mac build failure related to 'ar'

build.gradle

@@ -13,8 +13,21 @@ allprojects {
         evaluationDependsOn(":dependencies")
     }
 
-    ext.clangArgs = ["--sysroot=$sysrootDir", "-B$sysrootDir/usr/bin"]
     ext.clangPath = ["$llvmDir/bin"]
+    ext.clangArgs = []
+    final String binDir
+    if (isLinux()) {
+        binDir = "$gccToolchainDir/$gnuTriplet/bin"
+        ext.clangArgs.addAll([
+                "--sysroot=$gccToolchainDir/$gnuTriplet/sysroot",
+                "--gcc-toolchain=$gccToolchainDir"
+        ])
+    } else {
+        binDir = "$sysrootDir/usr/bin"
+        ext.clangArgs << "--sysroot=$sysrootDir"
+    }
+    ext.clangArgs << "-B$binDir"
+    ext.clangPath << binDir
 
     convention.plugins.execClang = new org.jetbrains.kotlin.ExecClang(project)
 

dependencies/build.gradle

@@ -78,9 +78,14 @@ task llvm(type: TgzNativeDep) {
     baseName = "clang+llvm-3.8.0-$target"
 }
 
-task sysroot(type: TgzNativeDep) {
-    String version = (target == 'linux-x86-64') ? '2' : '1'
-    baseName = "target-sysroot-$version-$target"
+if (isLinux()) {
+    task gccToolchain(type: TgzNativeDep) {
+        baseName = "target-gcc-toolchain-3-$target"
+    }
+} else {
+    task sysroot(type: TgzNativeDep) {
+        baseName = "target-sysroot-1-$target"
+    }
 }