gcc generates incorrect code

You'll also find that the x86 cpu ignores all the high bits of a shift count.

Oh definitions like:

static const uint32_t one = 1u;

are just completely pointless.

It is much clearer if you just write:

uint32_t makeMask(uint32_t count)
{
    return count >= 31 ? 0xffffffffu : (1u << count) - 1;
}

All the temporary variables just make it harder to read.

(That isn't quite your function, but is closer to the one you wanted!)