Make <isc/endian.h> detect GNU rather than Linux

Make <isc/endian.h> detect GNU rather than Linux

Instead of only supporting Linux, try making <isc/endian.h> support
other GNU platforms as well.  Since some compilers define __GNUC__ on
BSDs (e.g. Clang on FreeBSD), move the relevant section to the bottom of
the platform-specific part of <isc/endian.h>, so that it only gets
evaluated when more specific platform determination criteria are not
met.  Also include <byteswap.h> so that any byte-swapping macros which
may be defined in that file on older platforms are used in the fallback
definitions of the nonstandard hto[bl]e{16,32,64}() and
[bl]e{16,32,64}toh() conversion functions.

(cherry picked from commit a98c7408fc5c322a21548655d042919390edcf7b)