Add a FuncEnvironment::make_indirect_sig() callback.

The function environment is now expected to keep track of the function
signatures in the module, and it is asked to generate Cretonne
signatures to be used for indirect calls.

The combination of make_indirect_sig() and translate_call_indirect()
callbacks allow the runtime to insert additional function arguments for
indirect calls such as vmctx pointers and CFI-style signature identifiers.

cranelift/wasmtests/icall.wast

@@ -0,0 +1,7 @@
+(module
+  (type $ft (func (param f32) (result i32)))
+  (func $foo (export "foo") (param i32 f32) (result i32)
+    (call_indirect $ft (get_local 1) (get_local 0))
+  )
+  (table (;0;) 23 23 anyfunc)
+)