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/runtime_cortexm.go

@@ -8,19 +8,19 @@ import (
 )
 
 //go:extern _sbss
-var _sbss unsafe.Pointer
+var _sbss [0]byte
 
 //go:extern _ebss
-var _ebss unsafe.Pointer
+var _ebss [0]byte
 
 //go:extern _sdata
-var _sdata unsafe.Pointer
+var _sdata [0]byte
 
 //go:extern _sidata
-var _sidata unsafe.Pointer
+var _sidata [0]byte
 
 //go:extern _edata
-var _edata unsafe.Pointer
+var _edata [0]byte
 
 func preinit() {
 	// Initialize .bss: zero-initialized global variables.