Fix flatcc + pybindings linking on linux (#2467)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2467

On some linux systems, the pybindings build would fail with

`relocation R_X86_64_32 against '.rodata' can not be used when making a shared object; recompile with -fPIC`

Digging into it further, I also noticed that etdump was depending on `libflatcc` (the compiler) instead of `libflatccrt` (the runtime).

Fix that dep, then mark `flatccrt` as positition-independent (thus adding `-fPIC` on appropriate systems) to fix the pybindings build.

Reviewed By: tarun292, larryliu0820

Differential Revision: D54966723

fbshipit-source-id: 4e780082d900c21d8f2aa9cc8a7830d32915f463