Use the x64 host toolchain for x86 target gen_snapshot only on Linux

lib/snapshot/BUILD.gn

@@ -24,7 +24,7 @@ compiled_action("generate_snapshot_bin") {
   # platform is 32-bit.  Override this to create a 64-bit gen_snapshot for x86
   # targets because some host platforms may not support 32-bit binaries.
   tool = "//third_party/dart/runtime/bin:gen_snapshot_host_targeting_host"
-  if (target_cpu == "x86") {
+  if (target_cpu == "x86" && host_os == "linux") {
     toolchain = "//build/toolchain/$host_os:clang_x64"
   }