cmake: Move bpf-static and bpf-shared targets lower in file
cmake: Move bpf-static and bpf-shared targets lower in file

Move the definitions lower in the file so we can reuse some variables in
the next commit.

brendangregg/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc