ARM64 backend, part 8 / 11: integration.

This patch ties together the new backend infrastructure with the
existing Cranelift codegen APIs.

With all patches in this series up to this patch applied, the ARM64
compiler is now functional and can be used. Two uses of this
functionality -- filecheck-based tests and integration into wasmtime --
will come in subsequent patches.

cranelift/codegen/src/isa/test_utils.rs

@@ -0,0 +1,83 @@
+use crate::binemit::{Addend, CodeOffset, CodeSink, Reloc};
+use crate::ir::Value;
+use crate::ir::{ConstantOffset, ExternalName, Function, JumpTable, Opcode, SourceLoc, TrapCode};
+use crate::isa::TargetIsa;
+
+use alloc::vec::Vec;
+use std::string::{String, ToString};
+
+pub struct TestCodeSink {
+    bytes: Vec<u8>,
+}
+
+impl TestCodeSink {
+    /// Create a new TestCodeSink.
+    pub fn new() -> TestCodeSink {
+        TestCodeSink { bytes: vec![] }
+    }
+
+    /// This is pretty lame, but whatever ..
+    pub fn stringify(&self) -> String {
+        let mut s = "".to_string();
+        for b in &self.bytes {
+            s = s + &format!("{:02X}", b).to_string();
+        }
+        s
+    }
+}
+
+impl CodeSink for TestCodeSink {
+    fn offset(&self) -> CodeOffset {
+        self.bytes.len() as CodeOffset
+    }
+
+    fn put1(&mut self, x: u8) {
+        self.bytes.push(x);
+    }
+
+    fn put2(&mut self, x: u16) {
+        self.bytes.push((x >> 0) as u8);
+        self.bytes.push((x >> 8) as u8);
+    }
+
+    fn put4(&mut self, mut x: u32) {
+        for _ in 0..4 {
+            self.bytes.push(x as u8);
+            x >>= 8;
+        }
+    }
+
+    fn put8(&mut self, mut x: u64) {
+        for _ in 0..8 {
+            self.bytes.push(x as u8);
+            x >>= 8;
+        }
+    }
+
+    fn reloc_block(&mut self, _rel: Reloc, _block_offset: CodeOffset) {}
+
+    fn reloc_external(
+        &mut self,
+        _srcloc: SourceLoc,
+        _rel: Reloc,
+        _name: &ExternalName,
+        _addend: Addend,
+    ) {
+    }
+
+    fn reloc_constant(&mut self, _rel: Reloc, _constant_offset: ConstantOffset) {}
+
+    fn reloc_jt(&mut self, _rel: Reloc, _jt: JumpTable) {}
+
+    fn trap(&mut self, _code: TrapCode, _srcloc: SourceLoc) {}
+
+    fn begin_jumptables(&mut self) {}
+
+    fn begin_rodata(&mut self) {}
+
+    fn end_codegen(&mut self) {}
+
+    fn add_stackmap(&mut self, _val_list: &[Value], _func: &Function, _isa: &dyn TargetIsa) {}
+
+    fn add_call_site(&mut self, _opcode: Opcode, _srcloc: SourceLoc) {}
+}