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/legalizer/libcall.rs

@@ -1,7 +1,7 @@
 //! Expanding instructions as runtime library calls.
 
 use crate::ir;
-use crate::ir::{get_libcall_funcref, InstBuilder};
+use crate::ir::{libcall::get_libcall_funcref, InstBuilder};
 use crate::isa::{CallConv, TargetIsa};
 use crate::legalizer::boundary::legalize_libcall_signature;
 use alloc::vec::Vec;