Description: MYISAM index corruption occurs for bulk insert
or repair table which involves "repair by sorting"
algorithm.

Analysis: The index corruption happens because of the
incorrect sorting done by "my_qsort2()". This happens
for a bulk insert with more than 450001959 rows or
repair table with more than 450001959 rows. In 5.7,
"my_qsort2()" is replaced by std::sort() as part of
Bug#26929724.

Fix:- Back ported Bug#26929724 fix to ensure MyISAM repair
by sorting algorithm uses std::sort().