[vm] Fix host-mac target-linux build.

TEST=ci
Cq-Include-Trybots: luci.dart.try:dart-sdk-mac-arm64-try,dart-sdk-mac-try,pkg-mac-release-arm64-try,pkg-mac-release-try
Change-Id: I8d827b5ee0ef9a0c25c9d09c2ea989dbb18fa2ea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435443
Reviewed-by: Derek Xu <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/virtual_memory.h

@@ -116,9 +116,9 @@ class VirtualMemory {
   // Assumes
   //   * `this` has RX protection and is codesigned.
   //   * `target` has RW protection, and is at least as large as `this`.
-#if defined(DART_TARGET_OS_MACOS)
+#if defined(DART_HOST_OS_MACOS)
   bool DuplicateRX(VirtualMemory* target);
-#endif  // defined(DART_TARGET_OS_MACOS)
+#endif  // defined(DART_HOST_OS_MACOS)
 
   // Returns the cached page size. Use only if Init() has been called.
   static intptr_t PageSize() {