[Flang][MLIR] Alter Fir.GlobalOp to print and lower external attributes

Fir.GlobalOp's currently do not respect attributes that
are applied to them, this change will do two things:

- Allow lowering of arbitrary attributes applied to
Fir.GlobalOp's to LLVMGlobalOp's during CodeGen
- Allow printing and parsing of arbitrarily applied attributes

This allows applying other dialects attributes (or other
fir attributes) to fir.GlobalOps on the fly and have them
exist in the resulting LLVM dialect IR or FIR IR.

Reviewer: jeanPerier

Differential Revision: https://reviews.llvm.org/D148352