cet: Pass -mshstk to compiler for tst-cet-legacy-10a[-static].c

cet: Pass -mshstk to compiler for tst-cet-legacy-10a[-static].c

Pass -mshstk to compiler to silence Clang:

In file included from ../sysdeps/x86_64/tst-cet-legacy-10a.c:2:
../sysdeps/x86_64/tst-cet-legacy-10.c:29:7: error: always_inline function '_get_ssp' requires target feature 'shstk', but would be inlined into function 'do_test' that is compiled without support for 'shstk'
   29 |   if (_get_ssp () != 0)
      |       ^

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>