[lldb] Set the default architecture also in buildDefault

In D89056 the default value for architecture was moved to `build` so that
all called functions see the same architecture value. It seems there are a
few functions that call buildDefault directly (and not via build), so
on some test configurations that set a custom arch value the architecture
value is no longer available.

This just adds the architecture code from build to buildDefault to get
the bots green again while I'm looking for a better solution.