mips: Avoid SCORE_EXTERN

Update #2559.

cpukit/score/cpu/mips/cpu.c

@@ -54,6 +54,10 @@
 #include <rtems/score/isr.h>
 #include <rtems/score/wkspace.h>
 
+#if CPU_HARDWARE_FP
+Context_Control_fp _CPU_Null_fp_context;
+#endif
+
 /*
 ** Exception stack frame pointer used in cpu_asm to pass the exception stack frame
 ** address to the context switch code.

cpukit/score/cpu/mips/rtems/score/cpu.h

@@ -590,7 +590,7 @@ typedef CPU_Interrupt_frame CPU_Exception_frame;
  *  _CPU_Context_Initialize.
  */
 
-SCORE_EXTERN Context_Control_fp  _CPU_Null_fp_context;
+extern Context_Control_fp _CPU_Null_fp_context;
 
 /*
  *  Nothing prevents the porter from declaring more CPU specific variables.