pkg/instance: use -fpermissive -w when building old syzkaller
pkg/instance: use -fpermissive -w when building old syzkaller

Since we can be building very old revisions for bisection here,
make the build as permissive as possible.
Newer compilers tend to produce more warnings also kernel headers may be broken, e.g.:
ebtables.h:197:19: error: invalid conversion from ‘void*’ to ‘ebt_entry_target*’

Update #501

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