runtime: fix external address declarations

This is the same problem as in
https://github.com/tinygo-org/tinygo/pull/605, but other targets also
suffer from it.

Discovered with the GBA target, but as pointed out in
https://bugs.llvm.org/show_bug.cgi?id=42881#c1 this appears to be a bug
in the way external globals are declared, not in LLVM. Therefore I
decided that fixing it everywhere would be the best thing to do.

src/runtime/arch_wasm.go

@@ -12,7 +12,7 @@ const GOARCH = "wasm"
 const TargetBits = 32
 
 //go:extern __heap_base
-var heapStartSymbol unsafe.Pointer
+var heapStartSymbol [0]byte
 
 //go:export llvm.wasm.memory.size.i32
 func wasm_memory_size(index int32) int32