[GOLD] add new method for computing a build ID

[Geoff Pike]

>> Motivation for the change: computing a build ID is currently a
>> sequential bottleneck. E.g., in the example I mentioned in my email
>> yesterday, a 24.5s link was spending 5.2s on the build ID computation.
>
> Have you tried linking with OpenSSL and using their optimized MD5 and
> SHA1 variants?  I bet it makes a huge difference, some cpus even have
> SHA1 and MD5 instructions.
>
> For example, on my machine we can SHA1 at a rate of 760MB/sec and
> MD5 at a rate of ~775MB/sec.

I thought about trying a faster version of what's already being done,
but felt that should be a lower priority than getting rid of the
sequential bottleneck.

thanks,

Geoff