llext: Linkable loadable extensions

Adds the linkable loadable extensions (llext) subsystem which provides
functionality for reading, parsing, and linking ELF encoded executable
code into a managed extension to the running elf base image.

A loader interface, and default buffer loader implementation,
make available to the llext subsystem the elf data. A simple management
API provide the ability to load and unload extensions as needed. A shell
interface for extension loading and unloading makes it easy to try.

Adds initial support for armv7 thumb built elfs with very specific
compiler flags.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>
Co-authored-by: Chen Peng1 <peng1.chen@intel.com>
Co-authored-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>