llext: (cosmetic) rename llext_copy_section() to llext_copy_region()

llext_copy_section() is a misnomer, as it is copying a region of memory,
not a section. Rename it to llext_copy_region() and use 'region' in the
internal variables to better reflect their purpose.

No functional change is introduced in this commit.

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