Post-push fix for a crash in json.json_functions_innodb on Solaris for
SPARC.

Json_scalar_holder stores a Json_scalar value in a char buffer that is
large enough to store values of any sub-class of Json_scalar. The char
buffer is however not aligned, and when one Json_scalar_holder was
changed from a member variable of the cmp_item_json class to a local
variable in cmp_item_json::cmp(), the alignment changed in a way that
caused the json_functions_innodb test to crash on SPARC.

The fix is to change the type of the buffer from char[] to the
Any_json_scalar union type, which has more information about the
actual type of data stored, and helps the compiler choose the proper
alignment.

Change-Id: I8c064c135766fe4f9e34e596ab6bdb77bbcb7c68