[patch] switch to xxhash for buildid recomputation

[Frank Ch. Eigler]

Hi -

> > OK, switching.  Would you like me to push it to the main branch?
> Yes please!

Thanks, done.

> [...]
> > % curl https://inbox.sourceware.org/.../raw | git am -
> > 
> > is happy processing 100% of that patch, except that it puts more of
> > the email prose into the commit text.
> 
> Right. It certainly isn't disastrous. Even b4 shazam <mid> will be able
> to apply it, but generates a really odd looking commit message. [...]
> [...] It just inhibits some automation.

You're right.  This factor is perhaps less important for others who
have commit access already (so email is more for review than for
precious master-copy transport).

- FChE