[#2531] Aix 7.1 support.

Problem?
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In AIX 7.1 python-package doesn't build as well as we need it to. No `_ctypes`, no fast math in `pycrypto`, no 64bit binaries.

Solution?
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Adapted the current build helpers to:
  * recognize AIX 7.1 and act accordingly
  * name the arch appropriately (it seems we were using a label of `ppc64`  for 32bit package)
  * compile with external `libffi`
  * compile with external `gmp`
  * various other minor fixes.

How to check?
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Please review changes (excluding the GMP commit).
Build the package with `./chevahbs build`.
Test the built package with `./chevahbs test`

reviewers: @adiroiban