tcg/ppc: Introduce prepare_host_addr

Merge tcg_out_tlb_load, add_qemu_ldst_label, tcg_out_test_alignment,
and some code that lived in both tcg_out_qemu_ld and tcg_out_qemu_st
into one function that returns HostAddress and TCGLabelQemuLdst structures.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>