target-ppc: implement stxvll instructions

target-ppc: implement stxvll instructions

stxvll: Store VSX Vector Left-justified with Length

Vector (8-bit elements) in BE/LE:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+--+--+
|“T”|“h”|“i”|“s”|“ ”|“i”|“s”|“ ”|“a”|“ ”|“T”|“E”|“S”|“T”|00|00|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+--+--+

Storing 14 bytes would result in following Little/Big-endian Storage:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+--+--+
|“T”|“h”|“i”|“s”|“ ”|“i”|“s”|“ ”|“a”|“ ”|“T”|“E”|“S”|“T”|FF|FF|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+--+--+

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>