Fix GN arm64 build, add msan support in GN.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 6d07d77000288590145384e9e90bce0a843fdfff..91634ab07d53f3a6df7521b35ad8dd29d1648094 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -22,10 +22,15 @@ v8_interpreted_regexp = false
 v8_object_print = false
 v8_postmortem_support = false
 v8_use_snapshot = true
-v8_target_arch = target_cpu
 v8_random_seed = "314159265"
 v8_toolset_for_d8 = "host"
 
+if (is_msan) {

[adamk, 2015/09/01 21:48:32]

Why is this arm64 only? My gyp-fu is weak, but I don't see a similar stanza in
the gyp files.

I'd appreciate a comment here, otherwise this looks like pure magic.

+  v8_target_arch = "arm64"
+} else {
+  v8_target_arch = target_cpu
+}
+
 # The snapshot needs to be compiled for the host, but compiled with
 # a toolchain that matches the bit-width of the target.
 #
@@ -1380,6 +1385,9 @@ source_set("v8_base") {
       "src/arm64/decoder-arm64.cc",
       "src/arm64/decoder-arm64.h",
       "src/arm64/decoder-arm64-inl.h",
+      "src/arm64/delayed-masm-arm64.cc",
+      "src/arm64/delayed-masm-arm64.h",
+      "src/arm64/delayed-masm-arm64-inl.h",
       "src/arm64/deoptimizer-arm64.cc",
       "src/arm64/disasm-arm64.cc",
       "src/arm64/disasm-arm64.h",