Revert "runtime/internal/syscall: use ABIInternal for Syscall6 on amd64"

Revert "runtime/internal/syscall: use ABIInternal for Syscall6 on amd64"

This reverts CL 401096. Grandparent CL 388477 breaks cmd/go
TestScript/cover_pkgall_runtime.

For #51087.
For #52472.

Change-Id: Ie82fe5f50975f66eb91fb0d01cd8bbbd0265eb4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/401634
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>