target-ppc: improve stxvw4x implementation

target-ppc: improve stxvw4x implementation

Manipulate data and store 8bytes instead of 4bytes.

Vector (32-bit elements):
+----------+----------+----------+----------+
| 00112233 | 44556677 | 8899AABB | CCDDEEFF |
+----------+----------+----------+----------+

Store results in following:

Big-Endian Storage
+-------------+-------------+-------------+-------------+
| 00 11 22 33 | 44 55 66 77 | 88 99 AA BB | CC DD EE FF |
+-------------+-------------+-------------+-------------+

Little-Endian Storage
+-------------+-------------+-------------+-------------+
| 33 22 11 00 | 77 66 55 44 | BB AA 99 88 | FF EE DD CC |
+-------------+-------------+-------------+-------------+

Signed-off-by: Nikunj A Dadhania <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: David Gibson <[email protected]>