target/sparc: Factor out the body of sparc_cpu_unassigned_access()

Currently the SPARC target uses the old-style do_unassigned_access
hook.  We want to switch it over to do_transaction_failed, but to do
this we must first remove all the direct calls in ldst_helper.c to
cpu_unassigned_access().  Factor out the body of the hook function's
code into a new sparc_raise_mmu_fault() and call it from the hook and
from the various places that used to call cpu_unassigned_access().

In passing, this fixes a bug where the code that raised the
MMU exception was directly calling GETPC() from a function that
was several levels deep in the callstack from the original
helper function: the new sparc_raise_mmu_fault() instead takes
the return address as an argument.

Other than the use of retaddr rather than GETPC() and a comment
format fixup, the body of the new function has no changes from
that of the old hook function.

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: 20190801183012.17564-2-peter.maydell@linaro.org