PowerPC, fix gdb.base/retval-large-struct.exp

PowerPC, fix gdb.base/retval-large-struct.exp

Support for printining non-trivial return values was recently added in
commit:

  commit a0eda3df5b750ae32576a9be092b361281a41787
  Author: Carl Love <cel@us.ibm.com>
  Date:   Mon Nov 14 16:22:37 2022 -0500

    PowerPC, fix support for printing the function return value for non-trivial values.

The functionality can now be used to fix gdb.base/retval-large-struct.exp.
The test just needs to be compiled with -fvar-tracking to enable GDB to
determine the address off the return buffer when the function is called.

The current output from the test:

34        return big_struct;
(gdb) PASS: gdb.base/retval-large-struct.exp: continue to breakpoint: Break in print_large_struct
finish
warning: Cannot determine the function return value.
Try compiling with -fvar-tracking.
Run till exit from #0  return_large_struct () at binutils-gdb-current/gdb/testsuite/gdb.base/retval-large-struct.c:34
main (argc=1, argv=0x7fffffffcd58) at binutils-gdb-current/gdb/testsuite/gdb.base/retval-large-struct.c:44
44        return 0;
Value returned has type: struct big_struct_t. Cannot determine contents
(gdb) FAIL: gdb.base/retval-large-struct.exp: finish from return_large_struct
testcase binutils-gdb-current/gdb/testsuite/gdb.base/retval-large-struct.exp completed in 1 seconds

This patch adds the command line argument -fvar-tracking to enable gdb to
determine the return vaule and thus fixing the test.

Patch tested on Power 10 with no regressions.