[linux_platform] Implement a bunch of missing functions necessaries to make Xenia build correctly under Linux

With this part fixed and a function readapted from the original codebase to fix the different signature, we should have a compiling Linux build now.

src/xenia/base/clock_posix.cc

@@ -53,4 +53,5 @@ uint64_t Clock::QueryHostUptimeMillis() {
   return host_tick_count_platform() * 1000 / host_tick_frequency_platform();
 }
 
+uint64_t Clock::QueryHostInterruptTime() { return host_tick_count_platform(); }
 }  // namespace xe

src/xenia/base/main_init_posix.cc

@@ -0,0 +1,46 @@
+/**
+******************************************************************************
+* Xenia : Xbox 360 Emulator Research Project                                 *
+******************************************************************************
+* Copyright 2023 Ben Vanik. All rights reserved.                             *
+* Released under the BSD license - see LICENSE in the root for more details. *
+******************************************************************************
+*/
+#include <gdk/gdk.h>
+#include <gtk/gtk.h>
+#include <xbyak/xbyak/xbyak_util.h>
+
+#include "xenia/ui/window_gtk.h"
+
+class StartupCpuFeatureCheck {
+ public:
+  StartupCpuFeatureCheck() {
+    Xbyak::util::Cpu cpu;
+    const char* error_message = nullptr;
+    if (!cpu.has(Xbyak::util::Cpu::tAVX)) {
+      error_message =
+          "Your CPU does not support AVX, which is required by Xenia. See "
+          "the "
+          "FAQ for system requirements at https://xenia.jp";
+    }
+    if (error_message == nullptr) {
+      return;
+    } else {
+      GtkDialogFlags flags = GTK_DIALOG_DESTROY_WITH_PARENT;
+      auto dialog =
+          gtk_message_dialog_new(nullptr, flags, GTK_MESSAGE_ERROR,
+                                 GTK_BUTTONS_CLOSE, "%s", error_message);
+      gtk_dialog_run(GTK_DIALOG(dialog));
+      gtk_widget_destroy(dialog);
+      exit(1);
+    }
+  }
+};
+
+// This is a hack to get an instance of StartupAvxCheck
+// constructed before any initialization code,
+// where the AVX check then happens in the constructor.
+// Ref:
+// https://reviews.llvm.org/D12689#243295
+__attribute__((
+    init_priority(101))) static StartupCpuFeatureCheck gStartupAvxCheck;