Preliminary cleanup of <sys/cdefs.h>.

We don't support anything other than Clang and GCC, and we don't support
GCC earlier than 4.9.

Move the various synonyms for __attribute__(__whatever__) together.

Fix a __STDC__VERSION__ (for __STDC_VERSION__) typo.

Drop support for BSD _ANSI_SOURCE and _C99_SOURCE; there's enough confusing
nonsense here already, and plenty of other ways to ask for obsolete standards.

There are plenty more problems here -- what I really want to do is rationalize
our treatment of __STDC_VERSION__ -- but let's get some of this easy stuff
out of the way first.

Bug: https://code.google.com/p/android/issues/detail?id=194631
Change-Id: I7526b9770fdc01f8a4667781b65e2fb08287b20b