target/ppc: Initialize lazy_tlb_flush correctly

target/ppc: Initialize lazy_tlb_flush correctly

ppc_tr_init_disas_context() correctly sets lazy_tlb_flush to true on
certain CPU models.  However, it leaves it uninitialized, instead of
setting it to false on all others.

It wasn't caught before now because we didn't have examples in the tests
that exercised this path.  However it can now be caught using clang's
undefined behaviour sanitizer and the sam460ex board.

Suggested-by: Peter Maydell <[email protected]>
Signed-off-by: David Gibson <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>