pc-bios/s390-ccw: Silence GCC 11 stringop-overflow warning

pc-bios/s390-ccw: Silence GCC 11 stringop-overflow warning

When building on Fedora 34 (gcc version 11.0.0 20210210) we get:

  In file included from pc-bios/s390-ccw/main.c:11:
  In function ‘memset’,
      inlined from ‘boot_setup’ at pc-bios/s390-ccw/main.c:185:5,
      inlined from ‘main’ at pc-bios/s390-ccw/main.c:288:5:
  pc-bios/s390-ccw/libc.h:28:14: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
     28 |         p[i] = c;
        |         ~~~~~^~~

The offending code is:

  memset((char *)S390EP, 0, 6);

where S390EP is a const address:

  #define S390EP 0x10008

The compiler doesn't know how big that pointed area is, so it assume that
its length is zero. This has been reported as BZ#99578 to GCC:
"gcc-11 -Warray-bounds or -Wstringop-overread warning when accessing a
pointer from integer literal"
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578

As this warning does us more harm than good in the BIOS code (where
lot of direct accesses to low memory are done), silence this warning
for all BIOS objects.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210422145911.2513980-1-philmd@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20210502174836.838816-4-thuth@redhat.com>
[thuth: Use the pre-existing cc-option macro instead of adding a new one]
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>