endian.h: Define uint{16,32,64}_t

The Draft Posix Issue 8 standard requires that these be defined. Define
them in the usual way that lets multiple headers define them. Opted to
not just use #include <stdint.h>, allowed by the draft, to be
conservative. Add notes about how we comply with Issue 8, and that we've
opted to define these only as macros, though the standard allows
functions, macros or both.

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