[Object][ELF] Simplify ELFObjectFile by using ELFType.

This simplifies the usage and implementation of ELFObjectFile by using ELFType
to replace:

<endianness target_endianness, std::size_t max_alignment, bool is64Bits>

This does complicate the base ELF types as they must now use template template
parameters to partially specialize for the 32 and 64bit cases. However these
are only defined once.