Kernel/aarch64: Implement stub for asm_signal_trampoline

This get us further into the boot process, since Process::initialize
does not crash anymore.

Kernel/Process.cpp

@@ -430,6 +430,17 @@ void signal_trampoline_dummy()
         :
         : "i"(Syscall::SC_sigreturn),
         "i"(offset_to_first_register_slot));
+#elif ARCH(AARCH64)
+    asm(
+        ".global asm_signal_trampoline\n"
+        "asm_signal_trampoline:\n"
+        // TODO: Implement this when we support userspace for aarch64
+        "wfi\n"
+        "\n"
+        ".global asm_signal_trampoline_end\n"
+        "asm_signal_trampoline_end: \n");
+#else
+#    error Unknown architecture
 #endif
 }