Allow a program to export a map for other programs to use. This enables
cross-program map sharing.

parent program syntax:
BPF_TABLE("array", int, int, shared, 10);
BPF_TABLE_EXPORT(shared);

child program syntax:
BPF_TABLE("extern", int, int, shared, 10);
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>