Compiled BPF programs must consist of a single contiguous code block,
meaning trying to call other function entry points (besides the
kernel-defined helpers) is not possible. The bcc frontend didn't
explicitly prohibit this, even though the program would fail to
compile/load. Add an explicit check and error message.

Fixes: #653
Signed-off-by: Brenden Blanco <bblanco@gmail.com>