target/sparc: Correctly handle bus errors in page table walks

Currently we use the ldl_phys() function to read page table entries.
With the unassigned_access hook in place, if these hit an unassigned
area of memory then the hook will cause us to wrongly generate
an exception with a fault address matching the address of the
page table entry.

Change to using address_space_ldl() so we can detect and correctly
handle bus errors and give them their correct behaviour of
causing a translation error with a suitable fault status register.

Note that this won't actually take effect until we switch the
over to using the do_translation_failed hook.

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: 20190801183012.17564-5-peter.maydell@linaro.org