[JITLink] Add target features to LinkGraph

[JITLink] Add target features to LinkGraph

This patch adds SubtargetFeatures to LinkGraph. Similar to Triple, some
targets might use this information while linking.

One example, and the reason this patch was written, is linker relaxation
on RISC-V: different relaxations are possible depending on if the C
extension is enabled.

Note that the features are stored as `std::vector<std::string>` to prevent a
public dependency on MC. There is still a private dependency to be able to
convert SubtargetFeatures to a vector.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D149522