[Help-glpk] Can I simulate step function in GLPK?

[xiaomi]

I am considering to use the property that the result of GLPK is
non-negative to simulate step function as follows:
for example: step function y=u(5);
minimize y
y<=M(x-5) , where M is a large number to simulate sharp slope.
y<=1;

It seems to imply that when y<=5, y>=M(x-5) && y>=0.
Does that work(fast?)? And is GLPK default to be non-negative result?

Thanks.