Initial forward-edge CFI implementation (#3693)

* Initial forward-edge CFI implementation

Give the user the option to start all basic blocks that are targets
of indirect branches with the BTI instruction introduced by the
Branch Target Identification extension to the Arm instruction set
architecture.

Copyright (c) 2022, Arm Limited.

* Refactor `from_artifacts` to avoid second `make_executable` (#1)

This involves "parsing" twice but this is parsing just the header of an
ELF file so it's not a very intensive operation and should be ok to do
twice.

* Address the code review feedback

Copyright (c) 2022, Arm Limited.

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

cranelift/codegen/src/machinst/mod.rs

@@ -168,6 +168,16 @@ pub trait MachInst: Clone + Debug {
     /// Is this a safepoint?
     fn is_safepoint(&self) -> bool;
 
+    /// Generate an instruction that must appear at the beginning of a basic
+    /// block, if any. Note that the return value must not be subject to
+    /// register allocation.
+    fn gen_block_start(
+        _is_indirect_branch_target: bool,
+        _is_forward_edge_cfi_enabled: bool,
+    ) -> Option<Self> {
+        None
+    }
+
     /// A label-use kind: a type that describes the types of label references that
     /// can occur in an instruction.
     type LabelUse: MachInstLabelUse;