[HACK] Export variables as __attribute((used)) to avoid being purged by LTO

It would be better to declare the variable as extern or in header files
This patch ensures compatibility with pre-LTO cores

Author: facchinm

src/Squawk.cpp

@@ -77,8 +77,8 @@ extern intptr_t squawk_register;
 extern uint16_t cia;
 
 // Exports
-osc_t osc[4];
-uint8_t pcm = 128;
+__attribute__((used)) osc_t osc[4];
+__attribute__((used)) uint8_t pcm = 128;
 
 // ProTracker period tables
 const uint16_t period_tbl[84] PROGMEM = {
@@ -371,7 +371,7 @@ void SquawkSynth::stop() {
 }
 
 // Progress module by one tick
-void squawk_playroutine() {
+__attribute__((used)) void squawk_playroutine() {
   static bool lockout = false;
 
   if(!order_count) return;

src/Squawk.h

@@ -117,7 +117,7 @@ extern void squawk_playroutine() asm("squawk_playroutine");
 // uses 132 cycles (not counting playroutine)
 //     ~1/3 CPU @ 44kHz on 16MHz
 #define SQUAWK_CONSTRUCT_ISR(TARGET_REGISTER) \
-uint16_t cia, cia_count; \
+__attribute__((used)) uint16_t cia, cia_count; \
 intptr_t squawk_register = (intptr_t)&TARGET_REGISTER; \
 ISR(TIMER1_COMPA_vect, ISR_NAKED) { \
   asm volatile( \