[patch] Simplify the datalayout strings by using the defaults

[Tim Northover]

> In any case, would you be OK with the default changing being an
> independent commit?

Absolutely.

I had a detailed look at the AArch64 and ARM ones and a glance at the
others, and it all seemed consistent. I'll be happy to really check
the others if no-one objects to the change on principle.

Cheers.

Tim.