[gn] Port more build flags for gn

BUG=chromium:474921
LOG=n
NOTRY=true

Review-Url: https://codereview.chromium.org/1983093002
Cr-Commit-Position: refs/heads/master@{#36293}

build_overrides/v8.gni

@@ -9,6 +9,20 @@ if (is_android) {
   import("//build/config/android/config.gni")
 }
 
+declare_args() {
+  # V8 generates code for this architecture.
+  v8_target_arch = target_cpu
+}
+
+if (((v8_target_arch == "ia32" ||
+      v8_target_arch == "x64" ||
+      v8_target_arch=="x87") &&
+     (is_linux || is_mac)) ||
+    (v8_target_arch == "ppc64" && is_linux)) {
+  v8_enable_gdbjit = true
+}
+
+v8_imminent_deprecation_warnings = true
 v8_optimized_debug = false
 v8_extra_library_files = []
 v8_experimental_extra_library_files = []