cmd/link: add and use new SymKind SFirstUnallocated
The linker sources in several places used SXREF to mark the first
SymKind which is not allocated in memory. This is cryptic.
Instead use SFirstUnallocated, following the example of the
existing SFirstWritable.
Change-Id: If326ad63027402699094bcc49ef860db3772f82a
Reviewed-on: https://go-review.googlesource.com/c/go/+/715623 Reviewed-by: Than McIntosh <thanm@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>