[libc] Mark setjmp and longjmp as __cdecl

src/arch/i386/include/setjmp.h

 #define JBLEN 6
 typedef unsigned long jmp_buf[JBLEN];
 
-extern int setjmp (jmp_buf env);
-extern void longjmp (jmp_buf env, int val);
+extern int __cdecl setjmp (jmp_buf env);
+extern void __cdecl longjmp (jmp_buf env, int val);
 
 #endif /* ETHERBOOT_SETJMP_H */