Fix GN arm64 build, add msan support in GN.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 6d07d77000288590145384e9e90bce0a843fdfff..1721544f98d1671cb6acd87cf048eb59d9e07d86 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -22,10 +22,18 @@ 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) {
+  # 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
+}
+
 # The snapshot needs to be compiled for the host, but compiled with
 # a toolchain that matches the bit-width of the target.
 #
@@ -1380,6 +1388,9 @@ 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",