Fix sign of Ada rational constants

My earlier patch commit 0c03db90 ("Use correct sign in get_mpz") was
(very) incorrect.  It changed get_mpz to check for a strict sign when
examining part of an Ada rational constant.  However, in Ada the
"delta" for a fixed-point type must be positive, and so the components
of the rational representation will be positive.

This patch corrects the error.  It also renames the get_mpz function,
in case anyone is tempted to reuse this code for another purpose.

Finally, this pulls over the test from the internal AdaCore test suite
that found this issue.