Make get_libcall_funcref pub(crate) (#1291)

* Make `get_libcall_funcref` `pub(crate)`

Closes https://github.com/bytecodealliance/cranelift/issues/1273.

Since get_libcall_funcref is only used internally by the verifier,
it doesn't make sense to have it be public. This will encourage users to
look elsewhere for `memcpy` (they should be looking at
https://docs.rs/cranelift-frontend/0.51.0/cranelift_frontend/struct.FunctionBuilder.html#method.emit_small_memcpy)

cranelift/codegen/src/ir/libcall.rs

@@ -106,7 +106,7 @@ impl LibCall {
 /// for `inst`.
 ///
 /// If there is an existing reference, use it, otherwise make a new one.
-pub fn get_libcall_funcref(
+pub(crate) fn get_libcall_funcref(
     libcall: LibCall,
     call_conv: CallConv,
     func: &mut Function,