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

[Tom Tromey]

>>>>> "Frank" == Frank Ch Eigler <fche@redhat.com> writes:

Frank> Here's the patch.  Is there interest in me adding xxhash configury and
Frank> formally submitting it?  Or interest in benchmarking with something
Frank> else?

Note there's already some code in gdbsupport/common.m4.

Tom