Fix compile warnings in backtrace

Part of #10688

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

Marlin/src/backtrace/backtrace.cpp

 void backtrace(void) {
 
   UnwindFrame btf;
-  uint32_t sp,lr,pc;
+  uint32_t sp = 0, lr = 0, pc = 0;
 
   // Capture the values of the registers to perform the traceback
   __asm__ __volatile__ (

Marlin/src/backtrace/unwarmbytab.cpp

  */
 static UnwResult UnwTabExecuteInstructions(const UnwindCallbacks *cb, UnwTabState *ucb) {
 
-  UnwResult err;
   int instruction;
   uint32_t mask;
   uint32_t reg;