don't reserve space for the .stack_store when IRQ_SAFE is defined

if IRQ_SAFE is defined, this variable is never used, so this saves a
tiny bit of memory.

src/pimp_mixer_arm.S

@@ -25,10 +25,9 @@
 // #define NO_MIXING
 // #define NO_CLIPPING
 
+#ifndef IRQ_SAFE
 .stack_store:
 .word 0
-
-#ifndef IRQ_SAFE
 .ime_store:
 .word 0
 #endif