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

[Nick Clifton]

Hi Frank,

> Sweet.  OK, got this full-featured patch going on the trybots, liking
> the new/modified test cases.  Please let me know if there's anything
> you'd like me to tweak:

A few small tweaks please:

   * Please could you add a line to the ld/NEWS file mentioning the
     support for the hash method (if enabled when the linker was
     built).  Maybe even mention how much faster it is. :-)

   * Similarly please could you update the description of the --build-id
     option in the ld/ld.texi file.

   * Mega minor code formatting nit:

> +      XXH3_state_t* state = XXH3_createState();

     Please can we have a space between the end of the function name and
     the opening parenthesis of its arguments.  ie:

 > +      XXH3_state_t* state = XXH3_createState ();

Thanks.

Cheers
   Nick