[flang] Fix implementation of Kahan summation (#116897)
In the runtime's implementation of floating-point SUM, the
implementation of Kahan's algorithm for increased precision is
incorrect. The running correction factor should be subtracted from each
new data item, not added to it. This fix ensures that the sum of 100M
random default real values between 0. and 1. is close to 5.E7.
See https://en.wikipedia.org/wiki/Kahan_summation_algorithm.
