[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.

llvm-svn: 172515