sys/targets: use -m31 gcc option on s390x arch for 32bit targets
sys/targets: use -m31 gcc option on s390x arch for 32bit targets

In ESA/S390 mode, the CPU is able to address only 31bit of
memory address space. GCC on s390x expects the flag -m31 instead
of -m32 for 32bit code.

Signed-off-by: Alexander Egorenkov <91da4175525e9dfd14770d775671f3c95d603858@ibm.com>

google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller