[ARM] Compute a target feature which corresponds to the ARM version.
Currently, the interaction between the triple, the CPU, and the
supported features is a mess: the driver edits the triple to indicate
the supported architecture version, and the LLVM backend uses this to
figure out what instructions are legal. This makes it difficult to
understand what's happening, and makes it impossible to LTO together two
modules with different computed architectures.
Instead of relying on triple rewriting to get the correct target
features, we should add the right target features explicitly.
Differential Revision: https://reviews.llvm.org/D45240