cmd/link: mangle ABI name for shared linkage

Currently, when ABI wrappers are used, we don't use ABI aliases.
One exception is shared linkage. When loading a shared library, if
a symbol has only one ABI, and the name is not mangled, we don't
know what ABI it is, so we have to use ABI aliases.

This CL makes it always mangle ABIInternal function name in shared
linkage, so we know what ABI to choose when loading a shared
library. And we now can fully stop using ABI aliases when ABI
wrappers are used.

Change-Id: Id15d9cd72a59f391f54574710ebba7dc44cb6e23
Reviewed-on: https://go-review.googlesource.com/c/go/+/315869


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>