Support local BPF calls in ELF mode

[Alan-Jowett]

BPF programs with local calls (calls to BPF sub-programs) should separate the main program and the sub-program into different sections in the ELF file.

Entry point BPF programs each go in their own section and BPF sub-programs go into the default ".text" section and then a relocation is added to bind the entry point BPF program to the BPF sub-program.