avr32: pass target arch to gcc

As suggested by Alexander Lukichev and Thomas Petazzoni on the mailing list,
only one of the two avr32 microarchitectures is relevant for Buildroot:
avr32 Linux implies the avr32b microarchitecure, as used in the ap7000.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>