[dev.link] cmd/link: resolve ABI aliases for external symbols
ABI alias references in Go symbols are resolved during
loadObjFull. But for external symbols they are not resolved. If
there is a reference from an external symbol to a Go ABIInternal
symbol, this reference will be invalid as it is not resolved.
The old code resolve ABI aliases in the deadcode pass. But the
new deadcode pass doesn't do it, as it works with indices instead
of Symbols. We do this in LoadFull.
This makes all internal cgo linking tests pass on Mach-O.