arch: add hidden config knobs for 32- and 64-bit support and select

Add new hidden config knobs that architectures can select to
say whether they support 32- or 64-bit variants, and which
they default to.

reporter_name="Harold Grovesteen"

reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

reporter_comment=""