tests/tcg/s390x: Use -nostdlib for softmmu tests

tests/tcg/s390x: Use -nostdlib for softmmu tests

The code currently uses -nostartfiles, but this does not prevent
linking with libc. On Fedora there is no cross-libc, so the linking
step fails.

Fix by using the more comprehensive -nostdlib (that's also what
probe_target_compiler() checks for as well).

Fixes: 503e549e441e ("tests/tcg/s390x: Test unaligned accesses to lowcore")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230131182057.2261614-1-iii@linux.ibm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>