tests: Test endian.h, byteswap.h, sys/endian.h and both endian.h and byteswap.h together

What's required and not required to be defined is complicated. Write
tests to enshrine it:
endian.h and sys/

tests: Test endian.h, byteswap.h, sys/endian.h and both endian.h and byteswap.h together

What's required and not required to be defined is complicated. Write
tests to enshrine it:
endian.h and sys/endian.h:
[bl]e{16,32,64}toh
hto[bl]e{16,32,64}
byteswap.h:
{__,}bswap_{16,32,64}
sys/endian.h:
{__,}bswap{16,32,64}
_BYTE_ORDER
_BIG_ENDIAN
_LITTLE_ENDIAN
_PDP_ENDIAN
endian.h:
__BYTE_ORDER
__BIG_ENDIAN
__LITTLE_ENDIAN
__PDP_ENDIAN
__FLOAT_WORD_ORDER

We also ensure that the sys/endian.h conditions hold true when
we include both endian.h and sys/endian.h in either order.

NOT TESTED: deprecated symbols, internal to glibc symbols

Sponsored by: Netflix
Discussed with: markj (made the changes he requested)
Differential Revision: https://reviews.freebsd.org/D32052

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