Move POSIX header files to Newlib

[Sebastian Huber]

Hello,

I had some time to work on the POSIX header file move to Newlib:

https://sourceware.org/ml/newlib/2017/msg00343.html

I hope this is enough to build Ada without an installed BSP. It helps to 
get rid of all header file hacks in libbsd.

-- 
Sebastian Huber, embedded brains GmbH

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