Add BPF helper mapping

BPF helpers can be very specific to kernel versions, and the set of
available helpers may differ between the system where DTrace was
compiled and the system where it is being used.

We add runtime checking of specific BPF helpers, with support for
possible fallback helpers.  E.g. if probe_read_user() is not found,
we can use probe_read() instead (though that may not guarantee
successful execution).

This can also be used to check whether certain helpers exist.  By
convention, when the BPF helper function id maps to 0 (BPF_FUNC_unspec),
the helper is known not to be supported on the runtime system.

We include bpf.h from a kernel recent enough to support all the BPF
features that DTrace can make use of.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Eugene Loh <eugene.loh@oracle.com>