fix incorrect assumption in gdb.ada/watch_arg

fix incorrect assumption in gdb.ada/watch_arg

The testcase is assuming that the parameter being watched isn't being
modified. But the way the test program is written, this is not true
at all.  So this changes fixes the code to still reference the variable,
but in a way that does not modify its value.

gdb/testsuite:

        * gdb.ada/watch_arg/pck.ads, gdb.ada/watch_arg/pck.adb: New files.
        * gdb.ada/watch_arg/watch.adb: Adjust code to avoid modification
        of parameter X in procedure Foo.