tcg-arm: Use ldrd/strd for appropriate qemu_ld/st64

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-arm: Rearrange slow-path qemu_ld/st

Instead of using a branch-call-branch sequence, arrange for a
call-branch sequence, using the ARM's conditional call insn.
This reduces the size of the slow-path within the TB, and makes
the GETPC_EXT implementation identical for TCG and not-TCG.

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