JitAsmCommon: Make CommonAsmRoutinesBase a struct

This is just used as a means of carting around routines. It's not meant
to directly have functionality embedded within it--this is the job of
the inheriting data structure--so we can just make this a basic struct.

Particularly given all the data members were public to begin with.

Source/Core/Core/PowerPC/JitCommon/JitAsmCommon.h

@@ -12,9 +12,8 @@ alignas(16) extern const float m_one[4];
 alignas(16) extern const float m_quantizeTableS[128];
 alignas(16) extern const float m_dequantizeTableS[128];
 
-class CommonAsmRoutinesBase
+struct CommonAsmRoutinesBase
 {
-public:
   const u8* enter_code;
 
   const u8* dispatcher_mispredicted_blr;