[pac][bti] Activate branch protection flag for ARM64

Verify if Chromium's flag was enabled and toggle the flag
for V8, enabling support for PAC (Pointer Authentication Code)
and BTI (Branch Target Identification).

Bug: v8:10026, chromium:1145581, chromium:919548
Change-Id: I7c40674d2f9c8512639a7320b491006697420e28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3072158
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76126}

BUILD.gn

@@ -452,6 +452,12 @@ if (v8_enable_shared_ro_heap == "") {
                              v8_enable_pointer_compression_shared_cage
 }
 
+# Check if it is a Chromium build and activate PAC/BTI if needed.
+if (build_with_chromium && v8_current_cpu == "arm64" &&
+    arm_control_flow_integrity == "standard") {
+  v8_control_flow_integrity = true
+}
+
 assert(!v8_disable_write_barriers || v8_enable_single_generation,
        "Disabling write barriers works only with single generation")