target/ppc: Move VSX vector with length storage access insns to decodetree.

target/ppc: Move VSX vector with length storage access insns to decodetree.

Moving the following instructions to decodetree specification :

        {l, st}xvl(l)           : X-form

The changes were verified by validating that the tcg-ops generated by those
instructions remain the same, which were captured using the '-d in_asm,op' flag.

Also added a new function do_ea_calc_ra to calculate the effective address :
EA <- (RA == 0) ? 0 : GPR[RA], which is now used by the above-said insns,
and shall be used later by (p){lx, stx}vp insns.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Chinmay Rath <rathc@linux.ibm.com>
[np: Fix 32-bit build]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>