Don't generate 128-bit atomics in fuzzing

This commit fixes some issues cropping up due to fuzzing 128-bit atomics
in `cranelift-{icache,fuzzgen}`. Not all backends support these
operations and x64 only supports them with a feature enabled, so for now
disable fuzzing until it's ready to fuzz later.

cranelift/fuzzgen/src/function_generator.rs

@@ -1051,6 +1051,12 @@ static OPCODE_SIGNATURES: Lazy<Vec<OpcodeSignature>> = Lazy::new(|| {
  (Opcode::FcvtFromSint, &[I8X16], &[F64X2]),
  (Opcode::FcvtFromSint, &[I16X8], &[F64X2]),
  (Opcode::FcvtFromSint, &[I32X4], &[F64X2]),
+ // Only supported on x64 with a feature at this time, so 128-bit
+ // atomics are not suitable to fuzz yet.
+ (Opcode::AtomicRmw, _, &[I128]),
+ (Opcode::AtomicCas, _, &[I128]),
+ (Opcode::AtomicLoad, _, &[I128]),
+ (Opcode::AtomicStore, &[I128, _], _),
  )
  })
  .filter(|(op, ..)| {