Common/CodeBlock: Namespace code under the Common namespace

Brings more common code under the Common namespace.

Source/Core/Common/Arm64Emitter.h

@@ -1126,7 +1126,7 @@ private:
   void UXTL(u8 src_size, ARM64Reg Rd, ARM64Reg Rn, bool upper);
 };
 
-class ARM64CodeBlock : public CodeBlock<ARM64XEmitter>
+class ARM64CodeBlock : public Common::CodeBlock<ARM64XEmitter>
 {
 private:
   void PoisonMemory() override

Source/Core/Common/CodeBlock.h

@@ -11,6 +11,8 @@
 #include "Common/CommonTypes.h"
 #include "Common/MemoryUtil.h"
 
+namespace Common
+{
 // Everything that needs to generate code should inherit from this.
 // You get memory management for free, plus, you can use all emitter functions without
 // having to prefix them with gen-> or something similar.
@@ -116,3 +118,4 @@ public:
     m_children.emplace_back(child);
   }
 };
+}  // namespace Common

Source/Core/Common/x64Emitter.h

@@ -1155,7 +1155,7 @@ public:
   }
 };  // class XEmitter
 
-class X64CodeBlock : public CodeBlock<XEmitter>
+class X64CodeBlock : public Common::CodeBlock<XEmitter>
 {
 private:
   void PoisonMemory() override