[libomptarget] Support BE ELF files in plugins-nextgen (#85246)

Code in plugins-nextgen reading ELF files is currently hard-coded to
assume a 64-bit little-endian ELF format. Unfortunately, this assumption
is even embedded in the interface between GlobalHandler and Utils/ELF
routines, which use ELF64LE types.

To fix this, I've refactored the interface to use generic types, in
particular by using (a unique_ptr to) ObjectFile instead of
ELF64LEObjectFile, and ELFSymbolRef instead of ELF64LE::Sym.

This allows properly templating over multiple ELF format variants inside
Utils/ELF; specifically, this patch adds support for 64-bit big-endian
ELF files in addition to 64-bit little-endian files.