[dev.link] cmd/link: let cgo import overwrite contentless data symbol
A contentless data symbol may be a declaration of a cgo-imported
variable, e.g.
//go:cgo_import_dynamic xxx
var xxx uintptr
In this case, we want to mark the symbol imported, instead of
defined with zero value.
We used to load cgo directives before loading the object file, so
we'll mark the symbol SDYNIMPORT first. But in newobj mode,
currently we load cgo directives later. Letting SDYNIMPORT
overwrite contentless data symbol makes it work in both ordering.