[CPU] Gate AVX non-fatal warning to macOS

(cherry picked from commit 64351bbd07f5d98657e2a825a40bd403a7c78725)
(cherry picked from commit 9b6a288fac396a9d4ccd0937c8b9c0995386cf11)

src/xenia/cpu/backend/x64/x64_backend.cc

@@ -231,11 +231,18 @@ bool X64Backend::Initialize(Processor* processor) {
   }
 
   Xbyak::util::Cpu cpu;
+#if XE_PLATFORM_MAC
   if (!cpu.has(Xbyak::util::Cpu::tAVX)) {
     XELOGW(
         "This CPU does not support AVX. Continuing anyway (performance and "
         "compatibility may be reduced).");
   }
+#else
+  if (!cpu.has(Xbyak::util::Cpu::tAVX)) {
+    XELOGE("This CPU does not support AVX. The emulator will now crash.");
+    return false;
+  }
+#endif
 
   // Need movbe to do advanced LOAD/STORE tricks.
   if (cvars::x64_extension_mask & kX64EmitMovbe) {

src/xenia/cpu/backend/x64/x64_emitter.cc

@@ -92,11 +92,19 @@ X64Emitter::X64Emitter(X64Backend* backend, XbyakAllocator* allocator)
       backend_(backend),
       code_cache_(backend->code_cache()),
       allocator_(allocator) {
+#if XE_PLATFORM_MAC
+  if (!cpu_.has(Xbyak::util::Cpu::tAVX)) {
+    XELOGW(
+        "This CPU does not support AVX. Continuing anyway (performance and "
+        "compatibility may be reduced).");
+  }
+#else
   if (!cpu_.has(Xbyak::util::Cpu::tAVX)) {
     XELOGW(
         "Your CPU does not support AVX, which is required by Xenia. See the "
         "FAQ for system requirements at https://xenia.jp");
   }
+#endif
 
   feature_flags_ = amd64::GetFeatureFlags();