meson: define 's390' for 's390x' when building BPF objects

meson: define 's390' for 's390x' when building BPF objects

The kernel headers match on __s390__ so the build fails

../src/nsresourced/bpf/userns_restrict/userns-restrict.bpf.c:159:6: error: Must specify a BPF target arch via __TARGET_ARCH_xxx
void BPF_KPROBE(userns_restrict_free_user_ns, struct work_struct *work) {
     ^
/usr/include/bpf/bpf_tracing.h:817:20: note: expanded from macro 'BPF_KPROBE'
        return ____##name(___bpf_kprobe_args(args));                        \
                          ^
/usr/include/bpf/bpf_tracing.h:797:41: note: expanded from macro '___bpf_kprobe_args'
                                        ^
/usr/include/bpf/bpf_helpers.h:195:29: note: expanded from macro '___bpf_apply'
                            ^
note: (skipping 2 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
/usr/include/bpf/bpf_tracing.h:789:72: note: expanded from macro '___bpf_kprobe_args1'
                                                                       ^
/usr/include/bpf/bpf_tracing.h:563:29: note: expanded from macro 'PT_REGS_PARM1'
                            ^
<scratch space>:125:6: note: expanded from here
 GCC error "Must specify a BPF target arch via __TARGET_ARCH_xxx"