[release-branch.go1.12] cmd/link: put shlib ".type" functions in internal ABI

These functions are compiler generated, and as such are only available
in the internal ABI. Doing this avoids generating an alias symbol.
Doing that avoids confusion between unmangled and mangled type symbols.

Updates #30768
Fixes #33040

Change-Id: I8aba3934ffa994b1a19fc442cfe3e05642792a25
Reviewed-on: https://go-review.googlesource.com/c/go/+/186278


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>