Decouple TargetFeature strings.

"arm" and "aarch64" support different sets of `target_feature` and
`simd_test` arguments, and for "arm", the set of features that can be
dynamically detected is different again. Restructure the generator code
to allow this to be expressed accurately (in future patches).

This implementation preserves the way that target features are specified
for shared intrinsics, because this has an impact on the generated
documentation. In particular, rustdoc cannot look inside
`cfg_attr(target_arch = ...)` tests, so we use unconditional
`target_feature` attributes where possible.