[Flang][Lower] Attach target_cpu and target_features attributes to MLIR functions (#78289)

This patch forwards the target CPU and features information from the
Flang frontend to MLIR func.func operation attributes, which are later
used to populate the target_cpu and target_features llvm.func
attributes.

This is achieved in two stages:

1. Introduce the `fir.target_cpu` and `fir.target_features` module
attributes with information from the target machine immediately after
the initial creation of the MLIR module in the lowering bridge.

2. Update the target rewrite flang pass to get this information from the
module and pass it along to all func.func MLIR operations, respectively
as attributes named `target_cpu` and `target_features`. These attributes
will be automatically picked up during Func to LLVM dialect lowering and
used to initialize the corresponding llvm.func named attributes.

The target rewrite and FIR to LLVM lowering passes are updated with the
ability to override these module attributes, and the `CodeGenSpecifics`
optimizer class is augmented to make this information available to
target-specific MLIR transformations.

This completes a full flow by which target CPU and features make it all
the way from compiler options to LLVM IR function attributes.