[arm] Force a CPU default in the config args defaults

Currently if the user does not specify a default CPU or architecture
the compiler provieds no default values in the spec defaults.  We can
try to work from TARGET_CPU_DEFAULT but pulling that into the driver
is a bit crufty and doesn't really work well with the general
spec-processing model.  A better way is to ensure that with_cpu is
always set appropirately during configure.  To avoid problems with the
multilib fragment processing we defer this until after we have
processed any required fragments before selecting the default.

	* config.gcc (arm*-*-*): Ensure both target_cpu_cname and with_cpu
	are set after handling multilib fragments.  Set target_cpu_default2
	from with_cpu.

From-SVN: r249291