Fix build.

The "set default variant to armv5te if none defined" in the ARM config
was applying to all architectures, but as it turns out only ARM builds
actually cared.  When I made a copy of the "set default" code to the x86
config, one or the other would win for builds that didn't specify a
variant.  Turns out x86 won, so sim and voles worked, but opal didn't.