weak: move internal/weak to weak, and update according to proposal

The updates are:
- API documentation changes.
- Removal of the old package documentation discouraging linkname.
- Addition of new package documentation with some advice.
- Renaming of weak.Pointer.Strong -> weak.Pointer.Value.

Fixes #67552.

Change-Id: Ifad7e629b6d339dacaf2ca37b459d7f903e31bf8
Reviewed-on: https://go-review.googlesource.com/c/go/+/628455
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/cmd/link/internal/loader/loader.go

@@ -2336,9 +2336,6 @@ var blockedLinknames = map[string][]string{
 	// coroutines
 	"runtime.coroswitch": {"iter"},
 	"runtime.newcoro":    {"iter"},
-	// weak references
-	"internal/weak.runtime_registerWeakPointer": {"internal/weak"},
-	"internal/weak.runtime_makeStrongFromWeak":  {"internal/weak"},
 	// fips info
 	"go:fipsinfo": {"crypto/internal/fips/check"},
 }