sys/targets: use g++ as preprocessor on freebsd
sys/targets: use g++ as preprocessor on freebsd

As per:
https://github.com/google/syzkaller/pull/844#issuecomment-443509014

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