cranelift: Add Bswap instruction (#1092) (#5147)

Adds Bswap to the Cranelift IR. Implements the Bswap instruction
in the x64 and aarch64 codegen backends. Cranelift users can now:
```
builder.ins().bswap(value)
```
to get a native byteswap instruction.

* x64: implements the 32- and 64-bit bswap instruction, following
the pattern set by similar unary instrutions (Neg and Not) - it
only operates on a dst register, but is parameterized with both
a src and dst which are expected to be the same register.

As x64 bswap instruction is only for 32- or 64-bit registers,
the 16-bit swap is implemented as a rotate left by 8.

Updated x64 RexFlags type to support emitting for single-operand
instructions like bswap

* aarch64: Bswap gets emitted as aarch64 rev16, rev32,
or rev64 instruction as appropriate.

* s390x: Bswap was already supported in backend, just had to add
a bit of plumbing

* For completeness, added bswap to the interpreter as well.

* added filetests and runtests for each ISA

* added bswap to fuzzgen, thanks to afonso360 for the code there

* 128-bit swaps are not yet implemented, that can be done later

cranelift/interpreter/src/step.rs

@@ -806,6 +806,7 @@ where
         Opcode::UshrImm => binary_unsigned(Value::ushr, arg(0)?, imm_as_ctrl_ty()?)?,
         Opcode::SshrImm => binary(Value::ishr, arg(0)?, imm_as_ctrl_ty()?)?,
         Opcode::Bitrev => assign(Value::reverse_bits(arg(0)?)?),
+        Opcode::Bswap => assign(Value::swap_bytes(arg(0)?)?),
         Opcode::Clz => assign(arg(0)?.leading_zeros()?),
         Opcode::Cls => {
             let count = if Value::lt(&arg(0)?, &Value::int(0, ctrl_ty)?)? {