BPF: add extern func to data sections if specified

This permits extern function (BTF_KIND_FUNC) be added
to BTF_KIND_DATASEC if a section name is specified.
For example,

-bash-4.4$ cat t.c
void foo(int) __attribute__((section(".kernel.funcs")));
int test(void) {
  foo(5);
  return 0;
}

The extern function foo (BTF_KIND_FUNC) will be put into
BTF_KIND_DATASEC with name ".kernel.funcs".

This will help to differentiate two kinds of external functions,
functions in kernel and functions defined in other bpf programs.

Differential Revision: https://reviews.llvm.org/D93563