logging: Change Z_LOG_MSG_STACK_CREATE() variable

"flags" is a common variable name and is more likely to be passed
to LOG_DBG() or LOG_ERR() as a parameter. Changing the name of
the local variable created in Z_LOG_MSG_STACK_CREATE() from "flags"
to "_options" make it less likely that someone will pass a parameter
of that name to LOG_DBG() or LOG_ERR() and have that passed value
occluded by the variable created in Z_LOG_MSG_STACK_CREATE.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>