avr32: Work around byteswap bug in gcc < 4.2

gcc versions earlier than 4.2 sign-extends the result of le16_to_cpu()
and friends when we implement __arch__swabX() using
__builtin_bswap_X(). Disable our arch-specific optimizations when those
gcc versions are being used.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>