cmd/link: resolve ABI alias for runtime.unreachableMethod

We redirect references to unreachable methods to
runtime.unreachableMethod. We choose to use ABIInternal symbol
for this, because runtime.unreachableMethod is a defined Go
function.

When linking against shared libraries, and ABI wrappers are not
enabled, the imported function symbols are all ABI0 and aliased
to ABIInternal. We need to resolve ABI alias in this case.

Change-Id: Idd64ef46ce0b5f54882ea0069ce0d59dc9b7a599
Reviewed-on: https://go-review.googlesource.com/c/go/+/319891


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>