cranelift: add i64.{ishl,ushr,ashr} libcalls.

These libcalls are useful for 32-bit platforms.

On x86_32 in particular, commit 4ec16fa0 added support for legalizing
64-bit shifts through SIMD operations. However, that legalization
requires SIMD to be enabled and SSE 4.1 to be supported, which is not
acceptable as a hard requirement.

cranelift/codegen/src/legalizer/mod.rs

@@ -35,7 +35,7 @@ mod table;
 use self::call::expand_call;
 use self::globalvalue::expand_global_value;
 use self::heap::expand_heap_addr;
-use self::libcall::expand_as_libcall;
+pub(crate) use self::libcall::expand_as_libcall;
 use self::table::expand_table_addr;
 
 enum LegalizeInstResult {