avr: fix corrupted .data byte

The stack and the .data section shared the first byte, leading to
corruption of the first byte of .data.

targets/avr.S

@@ -17,6 +17,9 @@ reset:
     out  0x3d, xl; SPL
     out  0x3e, xh; SPH
 
+    ; Add one to the stack pointer, so it doesn't point in the .data section.
+    push r0
+
     ; Initialize .data
 init_data:
     ldi  xl, lo8(_sdata)