[test] Check platform in exported_symbols_test.

llvm-nm for Linux and macOS needs to be for the right operating system
if the local checkout contains clang toolchains for other operating
systems.

Change-Id: I8de702474a6fa14f06b48e3e60756b3f414272f8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/342940
Commit-Queue: Jonas Termansen <sortie@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

runtime/tests/vm/dart/exported_symbols_test.dart

@@ -13,10 +13,14 @@ main() {
 
   var nm;
   for (var path in [
-    "buildtools/linux-arm64/clang/bin/llvm-nm",
-    "buildtools/linux-x64/clang/bin/llvm-nm",
-    "buildtools/mac-arm64/clang/bin/llvm-nm",
-    "buildtools/mac-x64/clang/bin/llvm-nm",
+    if (Platform.isLinux) ...[
+      "buildtools/linux-arm64/clang/bin/llvm-nm",
+      "buildtools/linux-x64/clang/bin/llvm-nm",
+    ],
+    if (Platform.isMacOS) ...[
+      "buildtools/mac-arm64/clang/bin/llvm-nm",
+      "buildtools/mac-x64/clang/bin/llvm-nm",
+    ],
   ]) {
     if (new File(path).existsSync()) {
       nm = path;