Auto merge of #2894 - ecnelises:aix, r=JohnTitor

Support AIX operating system

This PR 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.

This one should be successor of #2278 (and thanks to `@EGuesnet` 's initial work).

Author: bors