llext: optimize allocations for read-only sections

When loading an extension, read-only sections that do not have
relocations and whose data is accessible from the ELF buffer can be
directly mapped as-is in the extension memory. This avoids the need to
allocate and copy unmodified data from the ELF buffer to the LLEXT heap.

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