runtime, cmd/link: store type descriptor length, not end

runtime, cmd/link: store type descriptor length, not end

Storing the type descriptor length lets us save a relocation.
It also avoids a problem for Darwin dynamic linking.

For #6853
Fixes #77350

Change-Id: If5c94330fe10d75690325f3d0b0658060ef3eb2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/739681
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>