llext: read all section headers at once

This change reads all section headers at once, instead of reading them
one by one. This is more efficient and allows to further simplify the
code downstream.

The section headers are directly accessed from the file buffer if the
llext_peek() function is supported by the loader. Otherwise, they are
read into a buffer allocated on the heap and used only during the
llext_load() function.

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