cmd/link: fix incorrect constant for macho-o load command

The constant defined in macho.go for LC_LOAD_WEAK_DYLIB was
not correct, was 0x18 should have been 0x80000018. Switch
to the correct definition.

Fixes #32233.

Change-Id: I9fb660a3cfd5e8c451a64947258f7ead76d98c79
Reviewed-on: https://go-review.googlesource.com/c/go/+/178723


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>