cranelift: improve syscall error/oom handling in JIT module (#5173)

* cranelift: improve syscall error/oom handling in JIT module

The JIT module has several places where it `expect`s or `panic`s
on syscall or allocator errors. For example, `mmap` and `mprotect`
can fail if Linux `vm.max_map_count` is not high enough, and some
users may wish to handle this error rather than immediately
crashing.

This commit plumbs these errors upward as new `ModuleError`
types, so that callers of jit module functions like
`finalize_definitions` and `define_function` can handle them
(or just `unwrap()`, as desired).

* cranelift: Remove ModuleError::Syscall variant

Syscall errors can just be folded into the generic Backend error,
which is an anyhow::Error

* cranelift-jit: return io::ErrorKind::OutOfMemory for alloc failure

Just using `io::Error::last_os_error()` is not correct as global
allocator impls are not required to set errno

cranelift/jit/examples/jit-minimal.rs

@@ -78,7 +78,7 @@ fn main() {
     module.clear_context(&mut ctx);
 
     // Perform linking.
-    module.finalize_definitions();
+    module.finalize_definitions().unwrap();
 
     // Get a raw pointer to the generated code.
     let code_b = module.get_finalized_function(func_b);