Add support for ObjectFormat to TargetSpecificAttr

Looking through the code, I saw a FIXME on IFunc to switch it
to a target specific attribute. In looking through it, i saw that
the no-longer-appropriately-named TargetArch didn't support ObjectFormat
checking.

This patch changes the name of TargetArch to TargetSpecific
(since it checks much more than just Arch), makes "Arch" optional, adds
support for ObjectFormat, better documents the TargetSpecific type, and
changes IFunc over to a TargetSpecificAttr.

Differential Revision: https://reviews.llvm.org/D41303

llvm-svn: 321201