Fix the toolchain used to build the snapshots in GN.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 81e6fef0ced26530e6b7b346f550a3b0bd8694c6..f838262498e4a363282a3e2b0d0818df2112f21e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -18,10 +18,26 @@ v8_interpreted_regexp = false
 v8_object_print = false
 v8_postmortem_support = false
 v8_use_snapshot = true
+
+# TODO(GYP): We should be just target_cpu and not need v8_target_arch.

[Nico, 2015/03/11 13:47:48]

this reads grammared incorrectly

[jochen (gone - plz use gerrit), 2015/03/11 16:10:30]

also, we want to be able to e.g. compile for linux x64 as target_cpu but use
v8_target_arch arm64. This will run v8 with a simulator, allowing for better
asan coverage.

[Dirk Pranke, 2015/03/11 16:53:30]

Will fix :).
Sure.

 v8_target_arch = current_cpu
+
 v8_random_seed = "314159265"
 v8_toolset_for_d8 = "host"
 
+# The snapshot needs to be compiled for the host, but compiled with
+# a toolchain that matches the bit-width of the target.
+# TODO(GYP): For now we only support 32-bit little-endian builds from
+# an x86/x64 Linux host. We need to support the other architectures as well.
+if (target_os == "android" || target_os == "chromeos") {

[jochen (gone - plz use gerrit), 2015/03/11 16:10:30]

why this restriction?

[Dirk Pranke, 2015/03/11 16:53:30]

I'm not sure I understand the question. Which restriction are you referring to? 
The host must be linux restriction is there mostly to be sure we understand what
configurations are actually being called (i.e., I don't expect trying to build
android v8 from windows to work at this point, so I don't want people to try it
and spend time debugging weird things).

I'm happy to restructure it if you like.

[jochen (gone - plz use gerrit), 2015/03/11 18:32:28]

ok, maybe add a comment then that this is supposed to work for all target and
host os combinations that are supported by v8 (or chrome for that matter)

[Dirk Pranke, 2015/03/11 18:56:10]

That was the intent of the comment on lines 30-31, but perhaps it's not
completely clear.

I will reword it a bit.

[jochen (gone - plz use gerrit), 2015/03/11 18:58:14]

ah, that's probably enough. Maybe architectures/OSs?

+  assert(host_os == "linux" &&
+         (host_cpu == "x86" || host_cpu == "x64") &&
+         (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86"))
+  snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+} else {
+  snapshot_toolchain = default_toolchain
+}
+
 ###############################################################################
 # Configurations
 #
@@ -322,7 +338,7 @@ action("run_mksnapshot") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   deps = [
-    ":mksnapshot($host_toolchain)",
+    ":mksnapshot($snapshot_toolchain)",
   ]
 
   script = "tools/run.py"
@@ -332,7 +348,7 @@ action("run_mksnapshot") {
   ]
 
   args = [
-    "./" + rebase_path(get_label_info(":mksnapshot($host_toolchain)",
+    "./" + rebase_path(get_label_info(":mksnapshot($snapshot_toolchain)",
                                       "root_out_dir") + "/mksnapshot",
                        root_build_dir),
     "--log-snapshot-positions",
@@ -1428,7 +1444,7 @@ source_set("v8_libplatform") {
 # Executables
 #
 
-if (current_toolchain == host_toolchain) {
+if (current_toolchain == snapshot_toolchain) {
   executable("mksnapshot") {
     visibility = [ ":*" ]  # Only targets in this file can depend on this.