llext: refact: move ELF loading and linking to separate files

This commit moves ELF loading and linking code to separate files. This
is done to make the code more manageable and to make it easier to add
new features in the future.

No functional changes are introduced by this commit, except for a few
static functions now made public to allow this file split to occur.

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