Documentation: Describe the FDPIC module support.

FDPIC has no page, and the parts of it a reader has to get right are
spread across binfmt/Kconfig, the ELF loader and the ARM toolchain
definitions.

The page covers what an FDPIC module is and what it buys over the
position independent ELF support already in the tree, how the loader
places one, where shared libraries come from and how they are found,
which entry points resolve a function descriptor and the rules for
adding another, and how to build a module and a library with a toolchain
that can emit FDPIC.  A comparison table places it against NXFLAT and
PIC ELF, and the reference section records the object layout and the
relocations.

The known limitation is stated: global constructors are not run for a
module loaded through binfmt.

Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Author: casaroli