[Bug libc/16459] endian.h does not define byte-order conversion functions

[Ondřej Bílka]

There is flexibility that we could use a function-like macro instead
function. If this needs fixing then by changing pattern

 #   define htobe64(x) __bswap_64 (x)
 #   define htole64(x) (x)

to


 #   define htobe64 __bswap_64
 #   define htole64 __bkeep_64

where __bkeep_64 is a identity function like

static uint64_t
__bkeep_64 (uint64_t x)
{
  return x;
}