Allow public Triple deduction from ObjectFiles.

Move logic that allows for Triple deduction from an ObjectFile object
out of llvm-objdump.cpp into a public factory, found in the ObjectFile
class.

This should allow other tools in the future to use this logic without
reimplementation.

Patch by Mitch Phillips

Differential Revision: https://reviews.llvm.org/D37719

llvm-svn: 313605