Support for implicit context for helper functions

[saxena-anurag]

There are some program type specific helper functions where the helper function signature does not take program context has input. This causes an extra overhead for extensions to store and lookup the context in some per-cpu or per-thread storage. This has a performance hit, if it is a per-packet hook.

Allowing helper function implementation to also take context as argument (along with the arguments defined in helper prototype) will save the extensions from extra overhead of storage and lookups.

This change will have 2 parts:

1. For native mode, bpf2c change to support helper functions with implicit context.
2. For JIT / interpret mode, ubpf changes to generate x64 code with context also passed as argument to the helper function.