Make v8_target_arch a GN declare_arg().

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 27c1fae120a2e5d7301560245f8a258f40289bc4..c03487e9567b63c7986f4a20feb73008108004ce 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -70,13 +70,20 @@ declare_args() {
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
-if (is_msan) {
+declare_args() {
   # Running the V8-generated code on an ARM simulator is a powerful hack that
-  # allows the tool to see the memory accesses from JITted code. Without this
-  # flag, JS code causes false positive reports from MSan.
-  v8_target_arch = "arm64"
-} else {
-  v8_target_arch = target_cpu
+  # allows sanitizers and other tools to see the memory accesses from
+  # JITted code.
+  v8_target_arch = ""

[Michael Achenbach, 2016/05/24 06:39:32]

I've recently added this to v8's overrides:
https://code.google.com/p/chromium/codesearch#chromium/src/v8/build_overrides...

It should be removed there now.

I'm not so sure about the documentation. This seems sanitizer specific and
should go to the is_msan condition below. As main docu maybe:
"V8 generates code for this architecture. If v8_target_arch differs from
target_arch, a simulator will be run."

+}
+
+if (v8_target_arch == "") {
+  if (is_msan) {
+    # Without this flag, JS code causes false positive reports from MSan.
+    v8_target_arch = "arm64"
+  } else {
+    v8_target_arch = target_cpu
+  }
 }
 
 if (v8_use_snapshot && v8_use_external_startup_data) {