xen: Implement common byte{order,swap}.h

xen: Implement common byte{order,swap}.h

The current swab??() infrastructure is unnecessarily complicated, and can be
replaced entirely with compiler builtins.

All supported compilers provide __BYTE_ORDER__ and __builtin_bswap??().

Nothing in Xen cares about the values of __{BIG,LITTLE}_ENDIAN; just that one
of them is defined.  Therefore, centralise their definitions in xen/config.h

Signed-off-by: Lin Liu <lin.liu@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>