Fix doc warnings and link failures (#1948)

Also add configuration to CI to fail doc generation if any links are
broken. Unfortunately we can't blanket deny all warnings in rustdoc
since some are unconditional warnings, but for now this is hopefully
good enough.

Closes #1947

cranelift/filetests/src/function_runner.rs

@@ -39,7 +39,8 @@ pub struct SingleFunctionCompiler {
 
 impl SingleFunctionCompiler {
     /// Build a [SingleFunctionCompiler] from a [TargetIsa]. For functions to be runnable on the
-    /// host machine, this [TargetISA] must match the host machine's ISA (see [with_host_isa]).
+    /// host machine, this [TargetIsa] must match the host machine's ISA (see
+    /// [SingleFunctionCompiler::with_host_isa]).
     pub fn new(isa: Box<dyn TargetIsa>) -> Self {
         let trampolines = HashMap::new();
         Self { isa, trampolines }