internal/syscall: add AIX operating system
internal/syscall: add AIX operating system

This commit adds AIX operating system to internal/syscall package for ppc64
architecture.

Updates: #25893

Change-Id: I5c3a9d4403ca170a7e894e06e68b83387d09b816
Reviewed-on: https://go-review.googlesource.com/c/138718
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,CAFxX/go,josharian/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,josharian/go,nawawi/go,akutz/go,akutz/go,nawawi/go,golang/go,nawawi/go,nawawi/go,akutz/go,josharian/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,akutz/go,josharian/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,josharian/go,cloudflare/go,golang/go,josharian/go,cloudflare/go,josharian/go,golang/go