syscall: remove linknames to runtime symbols for aix/ppc64

Replaces //go:linkname by assembly functions for syscall
functions on aix/ppc64.
Since the new runtime internal ABI, this was triggering an error if
syscall.Syscall6 was called by others packages like x/sys/unix.
This commit should remove every future occurences of this problem.

Fixes #28769

Change-Id: I6a4bf77472ee1e974bdb76b27e74275e568f5a76
Reviewed-on: https://go-review.googlesource.com/c/153997


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>