tcg-arm: Fix 64-bit tlb load for pre-v6

Found by inspection, since the effect of the bug was simply to
send all memory ops through the slow path.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-arm: Use movi32 in exit_tb

Avoid the mini constant pool for armv7, and avoid replicating
the test for pre-v7.

Signed-off-by: Richard Henderson <rth@twiddle.net>