Fix double-building of v8 in GN builds when setting just v8_target_cpu.

snapshot_toolchain.gni

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 15 matching lines @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import("//build/config/v8_target_cpu.gni")
 
 declare_args() {
   # The snapshot needs to be compiled for the host, but compiled with
   # a toolchain that matches the bit-width of the target.
   v8_snapshot_toolchain = ""
 }
 
-# TODO(GYP): For now we only support 32-bit little-endian target builds from an
-# x64 Linux host. Eventually we need to support all of the host/target
-# configurations v8 runs on.
+# TODO(GYP): Fill in any missing host/target combinations.
 if (v8_snapshot_toolchain == "") {
-  if (host_cpu == "x64" && host_os == "linux") {
+  if (v8_current_cpu == v8_target_cpu && current_cpu == target_cpu) {
+    # Args that are specified by the user can end up having the same
+    # effect as args that are specified in a toolchain; for example,
+    # building on linux x64 with v8_target_cpu == "arm64" will use
+    # the //build/toolchain/linux:clang_x64 toolchain, but have the
+    # same effect as if you specified
+    #
+    #   custom_toolchain="//build/toolchain/linux:clang_x64_v8_arm64"
+    #
+    # As a result, if we didn't take that into account we could
+    # accidentally end up building two identical copies of v8.
+    v8_snapshot_toolchain = default_toolchain
+  } else if (host_cpu == "x64" && host_os == "linux") {
     if (current_cpu == "arm" || current_cpu == "mipsel" ||
         current_cpu == "x86") {
       _snapshot_cpu = "x86"
     } else {
       assert(current_cpu == "arm64" || current_cpu == "x64" ||
                  current_cpu == "mipsel64",
              "Need environment for this arch: $current_cpu")
       _snapshot_cpu = "x64"
     }
 
     if (v8_current_cpu != _snapshot_cpu) {
       _cpus = "${_snapshot_cpu}_v8_${v8_current_cpu}"
     } else {
       _cpus = _snapshot_cpu
     }
 
     if ((host_os == "linux" && current_os == "android") || is_clang) {
       v8_snapshot_toolchain = "//build/toolchain/linux:clang_${_cpus}"
     } else {
       v8_snapshot_toolchain = "//build/toolchain/linux:${_cpus}"
     }
   } else if (host_os == "mac" && current_os == "win") {
     assert(v8_current_cpu == current_cpu,
            "v8 target must match the regular target on this platform")
     v8_snapshot_toolchain = "//build/toolchain/mac:clang_$current_cpu"
   } else {
     v8_snapshot_toolchain = default_toolchain
   }
 }