testsuite: Use -fno-ipa-icf in gcc.dg/stack-check-2.c

testsuite: Use -fno-ipa-icf in gcc.dg/stack-check-2.c

One test failing with a -std=gnu23 default that I wanted to
investigate further is gcc.dg/stack-check-2.c.  The failures are

FAIL: gcc.dg/stack-check-2.c scan-tree-dump-not optimized "tail call"
FAIL: gcc.dg/stack-check-2.c scan-tree-dump-not tailc "tail call"

but it turns out the tail calls in question are not the ones that test
is actually checking for.  Rather, when () is interpreted as (void) in
C23 mode, ICF notices that certain functions are identical and so
turns test_indirect_2 into a tail call to text_indirect_1 and
test_indirect_casted_2 into a tail call to test_indirect_casted_1
(which it didn't do previously when one function used () and one used
(void)).

To avoid these spurious failures, make the test use -fno-ipa-icf
rather than relying on () and (void) giving different function types
to avoid ICF.

Tested for x86_64-pc-linux-gnu.

	* gcc.dg/stack-check-2.c: Use -fno-ipa-icf.