Support AIX operating system

This change adds rather complete definitions and declarations from AIX
system headers, with little modification to fit the assumption in some
crates or other targets in libc.

Currently only 64-bit PowerPC targets on AIX are proposed, so
definitions depending on pointer width are located in powerpc64 module.

Thanks to initial work from Etienne Guesnet, this patch is based on it
(#2278).

Author: ecnelises