llext: support alignment requirements for sections

This patch adds support for alignment requirements for sections in the
loader. The alignment requirements are taken from the ELF file and
checked when the section is mapped to a memory region and when the
region is copied to memory (or directly used from the ELF buffer).

Supporting a larger alignment requirement than the region's current
alignment is done by adjusting the region's start address. This needs
care to avoid detecting false overlaps with other regions and to ensure
that the region address and size reported by the inspection APIs are
correct. The extra offset needed to re-adjust these values is stored in
the 'sh_info' field of the region descriptor.

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