[WASM] Use prebuilt llvm-ar only on Linux

Author: kateinoigakukun

CMakeLists.txt

@@ -31,9 +31,6 @@ ENABLE_LANGUAGE(C)
 include(SwiftUtils)
 include(CheckSymbolExists)
 
-# WebAssembly: hack: use llvm-ar for creating static libraries; Ubuntu's GNU ar doesn't work with wasm-ld
-set(CMAKE_AR "${SWIFT_WASM_WASI_SDK_PATH}/bin/llvm-ar")
-
 #
 # User-configurable options that control the inclusion and default build
 # behavior for components which may not strictly be necessary (tools, examples,
@@ -853,6 +850,11 @@ if(swift_build_wasm AND NOT "${SWIFT_HOST_VARIANT_SDK}" STREQUAL "WASM")
   #  message(FATAL_ERROR "A Darwin or Linux host is required to build the Swift runtime for Android")
   #endif()
 
+  if ("${CMAKE_HOST_SYSTEM_NAME}" STREQUAL "Linux")
+    # WebAssembly: hack: use llvm-ar for creating static libraries; Ubuntu's GNU ar doesn't work with wasm-ld
+    set(CMAKE_AR "${SWIFT_WASM_WASI_SDK_PATH}/bin/llvm-ar")
+  endif()
+
   if("${SWIFT_SDK_WASM_ARCHITECTURES}" STREQUAL "")
     set(SWIFT_SDK_WASM_ARCHITECTURES wasm32)
   endif()