Some GCC compilation fixes (#3114)

* Fix 6-byte multichar literals

* Add `struct` to `e_ga_class::ga_s`

* Fix remaining wrong forward declares (struct/class)

* Replace `#include <string>` with `#include <cstring>`

* Guard FLT_EPSILON define to prevent redefinition

* Add missing `#include <cstring>` for direct cstring function usage

src/JSystem/JUtility/JUTCacheFont.cpp

@@ -7,6 +7,7 @@
 #include "JSystem/JKernel/JKRAram.h"
 #include <dolphin/gx.h>
 #include <stdint.h>
+#include <cstring>
 #include "angle_utils.h"
 
 JUTCacheFont::JUTCacheFont(ResFONT const* p_fontRes, u32 cacheSize, JKRHeap* p_heap) {