[AArch64] Add --with-tune configure flag

[Wilco Dijkstra]

Hi Richard,

> I specifically want to test generic SVE rather than SVE tuned for a
> specific core, so --with-arch=armv8.2-a+sve is the thing I want to test.

Btw that's not actually what you get if you use cc1 - you always get armv8.0,
so --with-arch doesn't work at all. The only case that appears to work in cc1
is --with-cpu as long as it is not --with-cpu=native.

Cheers,
Wilco