Simplify template parameters of Elf classes.

The ELF specification defines several types which differ between
32-bit ELF and 64-bit ELF.  We used to template all ELF-related
methods on all of those types which was very verbose.

This CL wraps all the types as typedefs in ElfTypes32 and ElfTypes64.
One of those wrappers is then used as the template parameter.

Change-Id: I65247c2c79d92a7c4799e988cf3e4a1b10eb4788