ppc test_dfp2 build fix for GCC 15

ppc test_dfp2 build fix for GCC 15

GCC 15 defaults to C23 which changes the meaning of unprototyped
functions, those declaring no arguments with (). Causing some errors:

test_dfp2.c:412:26: error: initialization of 'void (*)(void)' from incompatible pointer type 'void (*)(int)' [-Wincompatible-pointer-types]
  412 |                        { &_test_dscri,  "dscri",   dfp_2args_x1, 20, LONG_TEST, ">>", True},
      |                          ^
test_dfp2.c:412:26: note: (near initialization for 'dfp_two_arg_tests[0].test_func')
test_dfp2.c:110:13: note: '_test_dscri' declared here
  110 | static void _test_dscri (int shift)
      |             ^~~~~~~~~~~

test_dfp2.c:664:17: error: assignment to 'test_func_t' {aka 'void (*)(void)'} from incompatible pointer type 'test_func_main_t' {aka 'void (*)(int)'} [-Wincompatible-pointer-types]
  664 |    while ((func = all_tests[i].test_category)) {
      |                 ^
test_dfp2.c:237:16: note: 'test_func_t' declared here
  237 | typedef void (*test_func_t)();
      |                ^~~~~~~~~~~
test_dfp2.c:238:16: note: 'test_func_main_t' declared here
  238 | typedef void (*test_func_main_t)(int);
      |                ^~~~~~~~~~~~~~~~
test_dfp2.c:667:8: error: too many arguments to function 'func'; expected 0, have 1
  667 |       (*func)(has_vsx);
      |       ~^~~~~~ ~~~~~~~

Fix this by just explicitly compiling that code with -std=gnu99.