Refactor ELF type inference functions.

Previously, we initialized Config->EKind and Config->EMachine when
we instantiate ELF objects. That was not an ideal location to do that
because the logic was buried too deep inside a concrete logic.

This patch moves the code to the driver so that the initialization
becomes explicit.

Differential Revision: http://reviews.llvm.org/D21784

llvm-svn: 274089