target/sparc: Switch to do_transaction_failed() hook

Switch the SPARC target from the old unassigned_access hook to the
new do_transaction_failed hook.

This will cause the "if transaction failed" code paths added in
the previous commits to become active if the access is to an
unassigned address. In particular we'll now handle bus errors
during page table walks correctly (generating a translation
error with the right kind of fault status).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-id: 20190801183012.17564-8-peter.maydell@linaro.org