target/ppc: Update VMX storage access insns to use tcg_gen_qemu_ld/st_i128.

target/ppc: Update VMX storage access insns to use tcg_gen_qemu_ld/st_i128.

Updated instructions {l, st}vx to use tcg_gen_qemu_ld/st_i128,
instead of using 64 bits loads/stores in succession.
Introduced functions {get, set}_avr_full in vmx-impl.c.inc to
facilitate the above, and potential future usage.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Chinmay Rath <rathc@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>