target/ppc: improve avr64_offset() and use it to simplify get_avr64()/set_avr64()

target/ppc: improve avr64_offset() and use it to simplify get_avr64()/set_avr64()

By using the VsrD macro in avr64_offset() the same offset calculation can be
used regardless of the host endian. This allows get_avr64() and set_avr64() to
be simplified accordingly.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20190307180520.13868-6-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>