Try harder to detect unavailability of __builtin_mul_overflow(int64).

Try harder to detect unavailability of __builtin_mul_overflow(int64).

Commit c04d35f44 didn't quite do the job here, because it still allowed
the compiler to deduce that the function call could be optimized away.
Prevent that by putting the arguments and results in global variables.

Discussion: https://postgr.es/m/20171213213754.pydkyjs6bt2hvsdb@alap3.anarazel.de