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.

Sponsored by: Netflix

endian.h: Remove duplicate words

Reviewed by: emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/920

linux: For better compatibility, provide compatible endian.h

Add endian.h. This includes sys/endian.h and then adds extra defines
that glibc defines with double underscores for our
_{BIG,BYTE,LITTLE,PDP}_ENDIAN macros. We also define __FLOAT_WORD_ORDER
to be the same as _BYTE_ENDIAN since FreeBSD doesn't currently define
this, and the default with glibc is exactly this for our platforms.
Move common parts of endian.h and sys/endian.h into sys/_endian.h
to limit namespace pollution from endian.h

All this gives us good compatibility with Linux. There may be one or two
upstreams that haven't integrated the patches I tried to send up.

There are some minor differences:
o The extra glibc macros are not defined. These are all
controlled with either __ at the start, or only defined
when glibc is being built. We also don't define macros
that are used internally in glibc that would pollute
the namespace.
o For complete compatibility, this change must also be
paired with providing a glibc-compatible byteswap.h.

Sponsored by: Netflix
Reviewed by: mhorne, markj, jhb
Differential Revision: https://reviews.freebsd.org/D31962