compiler, runtime: optimize zero-sized allocations

Instead of referring to an unused global, use a constant value. This is
safe even when using `-gc=none` (since no actual memory gets allocated)
which wasn't the case before. It should also reduce binary size by a few
bytes for most programs.

src/runtime/arch_avr.go

@@ -9,6 +9,8 @@ const GOARCH = "arm" // avr pretends to be arm
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 8
 
+const zeroSizeAllocPtr uintptr = 16 // points into the register file or I/O sapce (which is memory mapped but shouldn't be accessed directly)
+
 const deferExtraRegs = 1 // the frame pointer (Y register) also needs to be stored
 
 const callInstSize = 2 // "call" is 4 bytes, "rcall" is 2 bytes