cranelift: Add big and little endian memory accesses to interpreter (#5893)

* Added `mem_flags` parameter to `State::checked_{load,store}` as the means
for determining the endianness, typically derived from an instruction.

* Added `native_endianness` property to `InterpreterState` as fallback when
determining endianness, such as in cases where there are no memory flags
avaiable or set.

* Added `to_be` and `to_le` methods to `DataValue`.

* Added `AtomicCas` and `AtomicRmw` to list of instructions with retrievable
memory flags for `InstructionData::memflags`.

* Enabled `atomic-{cas,rmw}-subword-{big,little}.clif` for interpreter run
tests.

cranelift/fuzzgen/src/lib.rs

@@ -145,7 +145,7 @@ impl fmt::Debug for TestCase {
             let returns = &self.main().signature.returns;
             let placeholder_output = returns
                 .iter()
-                .map(|param| DataValue::read_from_slice(&[0; 16][..], param.value_type))
+                .map(|param| DataValue::read_from_slice_ne(&[0; 16][..], param.value_type))
                 .map(|val| format!("{}", val))
                 .collect::<Vec<_>>()
                 .join(", ");