cmake: llext: avoid always compiling extensions

In CMake, unless explicitly disabled, the 'add_library()' and
'add_executable()' functions add their result to the ALL target.

This is undesirable for 'add_llext_target()', since it causes the source
files for all defined extensions to be compiled even when the extension
is not otherwise used by the build process.

Add 'EXCLUDE_FROM_ALL' where appropriate so that the object files are
only built when the extension is actually used by the project.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>