Handle ARMv6-J as an alias, instead of fake architecture

Summary:
This follows D14577 to treat ARMv6-J as an alias for ARMv6,
instead of an architecture in its own right.

The functional change is that the default CPU when targeting ARMv6-J
changes from arm1136j-s to arm1136jf-s, which is currently used as
the default CPU for ARMv6; both are, in fact, ARMv6-J CPUs.

The J-bit (Jazelle support) is irrelevant to LLVM, and it doesn't
affect code generation, attributes, optimizations, or anything else,
apart from selecting the default CPU.

Reviewers: rengolin, logan, compnerd

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D14755

llvm-svn: 253675