This is the same as the host toolchain, but with v8_current_cpu set to the target cpu.
--- a/build/toolchain/linux/unbundle/BUILD.gn.orig
+++ b/build/toolchain/linux/unbundle/BUILD.gn
@@ -39,3 +39,22 @@
     current_os = host_os
   }
 }
+
+gcc_toolchain("v8_snapshot_cross") {
+  cc = getenv("BUILD_CC")
+  cxx = getenv("BUILD_CXX")
+  ar = getenv("BUILD_AR")
+  nm = getenv("BUILD_NM")
+  ld = cxx
+
+  extra_cflags = getenv("BUILD_CFLAGS")
+  extra_cppflags = getenv("BUILD_CPPFLAGS")
+  extra_cxxflags = getenv("BUILD_CXXFLAGS")
+  extra_ldflags = getenv("BUILD_LDFLAGS")
+
+  toolchain_args = {
+    current_cpu = host_cpu
+    current_os = host_os
+    v8_current_cpu = target_cpu
+  }
+}
