target/ppc: Use tcg_gen_qemu_{ld,st}_i128 for LQARX, LQ, STQ

No need to roll our own, as this is now provided by tcg.
This was the last use of retxl, so remove that too.

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

target/ppc/cpu.h

@@ -1124,7 +1124,6 @@ struct CPUArchState {
                            /* used to speed-up TLB assist handlers */
 
     target_ulong nip;      /* next instruction pointer */
-    uint64_t retxh;        /* high part of 128-bit helper return */
 
     /* when a memory exception occurs, the access type is stored here */
     int access_type;