llext: add support for ELF init/fini arrays

Load the .preinit_array, .init_array and .fini_array sections in ELF
files. These sections are arrays of function pointers that are filled by
the compiler with the addresses of functions that need to be called at
startup or termination by the loader, such as C++ constructors and
destructors.

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