unix: set 100644 mode on mksyscall_aix_ppc64.go

Change mode of mksyscall_aix_ppc64.go to 644.

Change-Id: Icebc0944a739b58111f5420ec2f0850b348f1fee
Reviewed-on: https://go-review.googlesource.com/c/160918
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>