ada: Convert floating-point zero to machine representation

ada: Convert floating-point zero to machine representation

When statically evaluating floating-point expressions we convert the final
result to machine number. However, we skipped this conversion if the result was
zero.

This inconsistency was introduced when adding a warning for compile-time
evaluation that gives different result from a run-time evaluation, but left
when this warning was deconstructed. It causes a crash in GNATprove, which
expects all floating-point numbers in the GNAT AST to be in a machine
representation form.

gcc/ada/ChangeLog:

	* sem_eval.adb (Check_Non_Static_Context): Remove special handling of
	floating-point zero.