windows: scan globals conservatively

Scan globals conservatively by reading writable sections from the PE
header.

I'd like to get rid of needing to precisely scan globals eventually, and
this brings us one step closer. It also avoids a bug with ThinLTO on
Windows.

src/runtime/gc_none.go

@@ -11,6 +11,8 @@ import (
 	"unsafe"
 )
 
+const gcAsserts = false // perform sanity checks
+
 func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer
 
 func realloc(ptr unsafe.Pointer, size uintptr) unsafe.Pointer
@@ -38,3 +40,7 @@ func initHeap() {
 func setHeapEnd(newHeapEnd uintptr) {
 	// Nothing to do here, this function is never actually called.
 }
+
+func markRoots(start, end uintptr) {
+	// dummy, so that markGlobals will compile
+}