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

Because of the somewhat strange way default toolchains and custom
toolchains and user-specified arguments work in GN, if you did a v8
build that just set v8_target_cpu, you could end up building two
identical copies of v8 (see the comments in the change for more).

This CL identifies that case and fixes it.

R=machenbach@chromium.org
BUG=629825

Review-Url: https://codereview.chromium.org/2166173002
Cr-Commit-Position: refs/heads/master@{#37926}

snapshot_toolchain.gni

@@ -33,11 +33,21 @@ declare_args() {
   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"