Fix SymOffExp::toConstElem.

I am surprised and horrified at the same time that the test suite
(on platforms other than Windows, where the code path is triggered
in core.stdc.stdio) did not catch this at all.

The elemSize-dependent path probably doesn't make too much sense for
global variables, as it always refers to the total size of the global.
Should add a special case for arrays for clearer codegen in those cases.

GitHub: Fixes #477.