[Flang][Driver] Handle target CPU and features

This patch:

- Adds target-feature and target-cpu to FC1Options.
- Moves getTargetFeatures() from Clang.cpp to CommonArgs.cpp.
- Processes target cpu and features in the flang driver. Right now
  features are only added for AArch64/x86 because I only did basic
  testing on them but it should generally work for others as well.
  Option handling is similar to clang.
- Adds appropriate structures in TargetOptions and passes them to
  the target machine.

What's missing:

- Adding the CPU info and the features as attributes in the LLVM IR
  module.
- Processing target specific flags, e.g. SVE vector bits for AArch64,
  ABI etc.

Differential Revision: https://reviews.llvm.org/D137995

Change-Id: Ib081a74ea98617674845518a5d2754edba596418