runtime: port assembly files for aix/ppc64

This commit adds the change on asm_ppc64.s and tls_ppc64.s files for AIX
operating system.

R2 does not need to be set for aix/ppc64 since it should remain valid
througout Go execution, except after a call to a C function.
Moreover, g must always be saved on the tls as syscalls are made with
C functions.

Some modifications on asm_ppc64.s are due to AIX stack layout.

It also removes a useless part in asmcgocall which was done twice.

Change-Id: Ie037ab73da00562bb978f2d0f17fcdabd4a40aa2
Reviewed-on: https://go-review.googlesource.com/c/138735


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>