[libomptarget] Support BE ELF files in plugins-nextgen (#83976)

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 push all ELF specific
types into Utils/ELF. Specifically, this patch removes both the
getSymbol and getSymbolAddress routines and replaces them with a
single findSymbolInImage, which gets a StringRef identifying the
raw object file image as input, and returns a StringRef covering
the data addressed by the symbol (address and size) if found, or
std::nullopt otherwise.

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.