Cranelift: `fcvt_from_{u,s}int` not implemented for `i128` on AArch64

👋 Hey,

This came up when trying to fuzz #4884 on AArch64

### `.clif` Test Case

```
test run
target aarch64

function %a(i128) -> f32 {
block0(v0: i128):
v1 = fcvt_from_uint.f32 v0
return v1
}
; run: %a(0) == 0.0

function %c(i128) -> f64 {
block0(v0: i128):
v1 = fcvt_from_uint.f64 v0
return v1
}
; run: %c(0) == 0.0

function %signed_a(i128) -> f32 {
block0(v0: i128):
v1 = fcvt_from_sint.f32 v0
return v1
}
; run: %signed_a(0) == 0.0

function %signed_c(i128) -> f64 {
block0(v0: i128):
v1 = fcvt_from_sint.f64 v0
return v1
}
; run: %signed_c(0) == 0.0

```

### Steps to Reproduce

* `clif-util ./the-above.clif`

### Expected Results

The above tests to pass.

### Actual Results

```
Running `/home/ubuntu/git/wasmtime/target/debug/clif-util test ./lmao.clif`
thread 'worker #0' panicked at 'internal error: entered unreachable code: implemented in ISLE: inst = `v1 = fcvt_from_sint.f64 v0`, type = `Some(types::F64)`', cranelift/codegen/src/isa/aarch64/lower_inst.rs:40:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
ERROR cranelift_filetests::concurrent > FAIL: panicked in worker #0: internal error: entered unreachable code: implemented in ISLE: inst = `v1 = fcvt_from_sint.f64 v0`, type = `Some(types::F64)`
FAIL ./lmao.clif: panicked in worker #0: internal error: entered unreachable code: implemented in ISLE: inst = `v1 = fcvt_from_sint.f64 v0`, type = `Some(types::F64)`
1 tests
Error: 1 failure
```

This fails in the first function but all the functions in all test cases crash with a similar error.

### Versions and Environment

Cranelift version or commit: main
Operating system: Linux
Architecture: aarch64

### Extra Info

`cg_clif` works around this by emitting a libcall: https://github.com/bjorn3/rustc_codegen_cranelift/blob/fa7660440b2581c4c57d896d50e76da1515de6df/src/cast.rs#L50-L85