target-ppc: implement stxvl instruction

target-ppc: implement stxvl instruction

stxvl: Store VSX Vector with Length

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

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

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>