Allow specifying custom bottle architectures

This allows the user to build a custom bottle with the architecture of their choice, using the `--build-bottle=arch` option. When specified, both stdenv and superenv will use the specified architecture instead of the default oldest CPU.

This could be useful for users building custom bottles for their own taps. I wrote it for Tigerbrew, where canonical bottles are going to come in at least two flavours per OS (with and without Altivec), possibly more for some packages; this would enable building both bottles for an OS on the same hardware.

Ping @mikemcquaid