System: Rewrite EXE override/loading

Relies on POST=7 as a kernel initialization indicator, instead of
patching the BIOS.

Fixes EXE loading with OpenBIOS and PS2 BIOS, and fast boot getting
baked into save states.

src/core/cpu_core_private.h

@@ -7,6 +7,8 @@
 
 namespace CPU {
 
+void SetPC(u32 new_pc);
+
 // exceptions
 void RaiseException(Exception excode);
 void RaiseException(u32 CAUSE_bits, u32 EPC);