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>

crates/jit/src/code_memory.rs

@@ -40,7 +40,7 @@ pub struct Publish<'a> {
     pub obj: File<'a>,
 
     /// Reference to the entire `MmapVec` and its contents.
-    pub mmap: &'a [u8],
+    pub mmap: &'a MmapVec,
 
     /// Reference to just the text section of the object file, a subslice of
     /// `mmap`.
@@ -87,7 +87,7 @@ impl CodeMemory {
     /// After this function executes all JIT code should be ready to execute.
     /// The various parsed results of the internals of the `MmapVec` are
     /// returned through the `Publish` structure.
-    pub fn publish(&mut self) -> Result<Publish<'_>> {
+    pub fn publish(&mut self, enable_branch_protection: bool) -> Result<Publish<'_>> {
         assert!(!self.published);
         self.published = true;
 
@@ -159,7 +159,7 @@ impl CodeMemory {
             // read/execute, notably not using read/write/execute to prevent
             // modifications.
             self.mmap
-                .make_executable(text_range.clone())
+                .make_executable(text_range.clone(), enable_branch_protection)
                 .expect("unable to make memory executable");
 
             #[cfg(all(target_arch = "aarch64", target_os = "linux"))]