cmd/link: support weak binding on darwin

cmd/link: support weak binding on darwin

Symbols loaded from host files can have the N_WEAK_REF bit set,
which is used to instruct the loader to not fail if that symbol
can't be resolved.

The Go internal linker should honor this information by setting the
BIND_SYMBOL_FLAGS_WEAK_IMPORT flag in the corresponding bind table
entry.

Fixes #76023

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64-longtest,gotip-darwin-amd64_12,gotip-darwin-arm64_12,gotip-darwin-arm64_15,gotip-darwin-arm64-longtest,gotip-darwin-amd64_14
Change-Id: Id2cef247ec7a9cb08455844f3c30ff874772bb7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/713760
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>