unix: add support for aix/ppc64, gc implementation

This commit ports unix package for aix/ppc64 with gc implementation.

In order to merge as much as possible gc and gccgo implementation, the
mksyscall_aix_ppc64.pl creates three files.

zsyscall_aix_ppc64.go is common for both implementation. It has the main
syscall function and handles the pointer creations, type conversions
and the error if needed. It calls a function "call..." which is
available for each implementation.

zsyscall_aix_ppc64_gc.go is the gc part. It implements "call..."
functions using //go:cgo_import_dynamic and //go:linkname. It is
based on syscall.syscall6 function.

zsyscall_aix_ppc64_gccgo.go is the gccgo part. It implements "call..."
functions using cgo and C functions.

Some unavailable syscalls were also removed from the previous
implementation.

For aix/ppc, the script is left unchanged as aix/ppc won't be
implemented inside gc.

Change-Id: I3701095df31517c66f95874ba8e682967993090b
Reviewed-on: https://go-review.googlesource.com/c/143117
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>