sys/targets: fix -static-pie for s390x arch
sys/targets: fix -static-pie for s390x arch

On s390x, position independent code needs to be built with -fPIC or
-fPIE for pie executables.

How to reproduce on s390x
-------------------------

$ git clone https://github.com/google/syzkaller.git
$ cd syzkaller
$ make executor
go list -f '{{.Stale}}' ./sys/syz-sysgen | grep -q false || go install ./sys/syz-sysgen
make .descriptions
make[1]: '.descriptions' is up to date.
mkdir -p ./bin/linux_s390x
gcc -o ./bin/linux_s390x/syz-executor executor/executor.cc \
        -O2 -pthread -Wall -Werror -Wparentheses -Wunused-const-variable \
        -Wframe-larger-than=16384 -Wno-stringop-overflow -Wno-array-bounds \
        -Wno-format-overflow -static-pie  -DGOOS_linux=1 -DGOARCH_s390x=1 \
        -DHOSTGOOS_linux=1 -DGIT_REVISION=\"2159e4d29f58ffa1107fc2213dbc87185ff4498f\"
/usr/bin/ld: read-only segment has dynamic relocations

Signed-off-by: Alexander Egorenov <352111b974d96d0f589c4a7c3a704200b81908f7@gmail.com>

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