tests/tcg/s390x: Test unaligned accesses to lowcore

Add a small test to avoid regressions.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220725223746.227063-3-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>