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/filetests/filetests/runtests/i128-bswap.clif

@@ -0,0 +1,12 @@
+test interpret
+
+function %bswap_i128(i128) -> i128 {
+block0(v0: i128):
+    v1 = bswap v0
+    return v1
+}
+; run: %bswap_i128(0) == 0
+; run: %bswap_i128(1) == 0x01000000_00000000_00000000_00000000
+; run: %bswap_i128(0x12345678_9ABCDEF0_CAFEF00D_F00DCAFE) == 0xFECA0DF0_0DF0FECA_F0DEBC9A_78563412
+; run: %bswap_i128(-2) == 0xFEFFFFFF_FFFFFFFF_FFFFFFFF_FFFFFFFF
+