tests/tcg/s390x: Allow specifying extra QEMU options on the command line

tests/tcg/s390x: Allow specifying extra QEMU options on the command line

The use case for this is `make check-tcg EXTFLAGS="-accel kvm"`,
which allows validating the system TCG testcases on real hardware.
EXTFLAGS name is borrowed from tests/tcg/xtensa/Makefile.softmmu-target.
While at it, use += instead of = in order to be consistent with the
other architectures.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240522184116.35975-1-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>