pc-bios/s390-ccw/Makefile: Compile with -std=gnu99, -fwrapv and -fno-common

pc-bios/s390-ccw/Makefile: Compile with -std=gnu99, -fwrapv and -fno-common

The main QEMU code is compiled with -std=gnu99, -fwrapv and -fno-common.
We should use the same flags for the s390-ccw bios, too, to avoid that
we get different behavior with different compiler versions that changed
their default settings in the course of time (it happened at least with
-std=... and -fno-common in the past already).

While we're at it, also group the other flags here in a little bit nicer
fashion: Move the two "-m" flags out of the "-f" area and specify them on
a separate line.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20200806105349.632-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>