executor: update KCOV support for FreeBSD
executor: update KCOV support for FreeBSD

KCOV support has been added to FreeBSD in r342962. Use
the header file and update the code to latest changes
introduced in the review process.

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