[Various] libc/2757: Multiple <bits/byteswap.h> erroneus include

[Roland McGrath]

> Shouldn't we add multiple include markers around the files in
> <bits/*>?  Or what should be done?

For this one, yes.  That's what's done in others that are included by more
than one header (e.g. bits/sigcontext.h).  For most of these headers there
should be no need since they should be included only by their corresponding
sysdeps-independent header.