[patch rfc]: use xxhash() in ld build-id computation, incl. benchmark

[Frank Ch. Eigler]

Hi -

> > Here's the patch.  Is there interest in me adding xxhash configury and
> > formally submitting it?
> 
>   Yes please.  Allowing distributions to choose if they want to support
>   this hashing method makes sense to me.  So providing them with a
>   configure option to do so is, imho, the right way to go.

OK.  For testing, any suggestions?  The ld/tests/build-id.exp test case
just tickles =md5 mode, but it's not obvious how to conditionalize it to
the others.  What about having ld list the supported hash styles on its
--help output, and having dejagnu parse that for md5/sha1/xx/etc. ?

- FChE