llext: fix a confusion between section indices

A common pattern is used throughout llext.c:

ext->mem[ldr->sect_map[sym.st_shndx]]

where ldr->sect_map[sym.st_shndx] actually contains indices from
enum llext_section but ext->mem[] is indexed, using enum llext_mem
values. Fix this by changing ldr->sect_map[] to actually contain
enum llext_mem values.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>