Common: Add header for compiler-specifics

Instead of globbing things under an ambiguous Common.h header, move
compiler-specifics over to Compiler.h. This gives us a dedicated home
for anything related to compilers that we want to make functional across
all compilers that we support.

This moves us a little closer to eliminating Common.h entirely.

Source/Core/Common/Compiler.h

@@ -0,0 +1,17 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#if defined(__GNUC__) || __clang__
+// Disable "unused function" warnings for the ones manually marked as such.
+#define UNUSED __attribute__((unused))
+#else
+// Not sure MSVC even checks this...
+#define UNUSED
+#endif
+
+#ifndef _WIN32
+#define __forceinline inline __attribute__((always_inline))
+#endif