tilegx: Fix infinite loop in gen-mul-tables generator

Since around GCC 10, the condition `j < (INTMAX_MAX / 10)' will get
optimized into `j != 922337203685477580', which will result in an
infinite loop for certain inputs of `j'.

Copy the condition already used by the -DTILEPRO generator code, which
doesn't fall into this trap.

gcc/ChangeLog:

	* config/tilepro/gen-mul-tables.cc (tilegx_emit): Adjust loop
	condition to avoid overflow.