Fix stdlib/tst-setcontext.c for GCC 12 -Warray-compare

Building stdlib/tst-setcontext.c fails with GCC mainline:

tst-setcontext.c: In function 'f2':
tst-setcontext.c:61:16: error: comparison between two arrays [-Werror=array-compare]
   61 |   if (on_stack < st2 || on_stack >= st2 + sizeof (st2))
      |                ^
tst-setcontext.c:61:16: note: use '&on_stack[0] < &st2[0]' to compare the addresses

The comparison in this case is deliberate, so adjust it as suggested
in that note.

Tested with build-many-glibcs.py (GCC mainline) for aarch64-linux-gnu.