fix: valid compiler optimization may fail the test

fix: valid compiler optimization may fail the test

cxa4001 may fail with "Exception not raised" when the compiler omits the
calls to To_Mapping, in accordance with 10.2.1(18/3):

  "If a library unit is declared pure, then the implementation is
  permitted to omit a call on a library-level subprogram of the library
  unit if the results are not needed after the call"

Using the result of both To_Mapping calls prevents the compiler from
omitting them.

"The corrected test will be available on the ACAA web site
(http://www.ada-auth.org/), and will be issued with the Modified Tests List
version 2.6K, 3.1DD, and 4.1GG."

gcc/testsuite/ChangeLog:

	* ada/acats/tests/cxa/cxa4001.a: Use function result.