[arm] Update default CPUs during configure

There are a couple of places in config.gcc where the default CPU is
still arm6, but that was removed as a supported CPU earlier this year.
This patch fixes those entries.

The default CPU for configurations that do not explicitly set a
default is now arm7tdmi (so assumes thumb is available).  Given that
StrongArm is on the deprecated list, this is a better default than we
had previously.

For NetBSD the default is StrongArm; this is the only remaining port
that uses the old ABI and really still carries support for non-thumb
based targets.

	PR target/86383
	* config.gcc (arm*-*-netbsdelf*): Default to StrongARM if no CPU
	specified to configure.
	(arm*-*-*): Use ARM7TDMI as the target CPU if no default provided.

From-SVN: r265420