[mlir] Add FP software implementation lowering pass: `arith-to-apfloat` (#166618)

This commit adds a new pass that lowers floating-point `arith`
operations to calls into the execution engine runtime library. Currently
supported operations: `addf`, `subf`, `mulf`, `divf`, `remf`.

All floating-point types that have an APFloat semantics are supported.
This includes low-precision floating-point types such as `f4E2M1FN` that
cannot execute natively on CPUs.

This commit also improves the `vector.print` lowering pattern to call
into the runtime library for floating-point types that are not supported
by LLVM. This is necessary to write a meaningful integration test.

The way it works is 

```mlir
func.func @full_example() {
  %a = arith.constant 1.4 : f8E4M3FN
  %b = func.call @foo() : () -> (f8E4M3FN)
  %c = arith.addf %a, %b : f8E4M3FN
  vector.print %c : f8E4M3FN
  return
}
```

gets transformed to

```mlir
func.func private @__mlir_apfloat_add(i32, i64, i64) -> i6
func.func @full_example() {
  %cst = arith.constant 1.375000e+00 : f8E4M3FN
  %0 = call @foo() : () -> f8E4M3FN
  // bitcast operand A to integer of equal width
  %1 = arith.bitcast %cst : f8E4M3FN to i8
  // zext A to i64
  %2 = arith.extui %1 : i8 to i64
  // same for operand B
  %3 = arith.bitcast %0 : f8E4M3FN to i8
  %4 = arith.extui %3 : i8 to i64
  // get the llvm::fltSemantics(f8E4M3FN) as an enum
  %c10_i32 = arith.constant 10 : i32
  // call the impl against APFloat in mlir_apfloat_wrappers
  %5 = call @__mlir_apfloat_add(%c10_i32, %2, %4) : (i32, i64, i64) -> i64
  // "cast" back to the original fp type
  %6 = arith.trunci %5 : i64 to i8
  %7 = arith.bitcast %6 : i8 to f8E4M3FN
  vector.print %7 : f8E4M3FN
}
```

Note, `llvm::fltSemantics(f8E4M3FN)` is emitted by the pattern each time
an `arith` op is transformed, thereby making the call to
`__mlir_apfloat_add` correct (i.e., no name mangling on type necessary).


RFC:
https://discourse.llvm.org/t/rfc-software-implementation-for-unsupported-fp-types-in-convert-arith-to-llvm/88785

---------

Co-authored-by: Matthias Springer <me@m-sp.org>

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