make variadic args explicit

Some abis, like the riscv one, treat
arguments differently depending on
whether they are variadic or not.
To prepare for the upcomming riscv
target, we change the variadic call
syntax and give meaning to the
location of the '...' marker.

  # new syntax
  %ret =w call $f(w %regular, ..., w %variadic)

By nature of their abis, the change
is backwards compatible for existing
targets.

doc/il.txt

@@ -751,7 +751,7 @@ single-precision floating point number `%f` into `%rs`.
     ARG :=
         ABITY VAL  # Regular argument
       | 'env' VAL  # Environment argument (first)
-      | '...'      # Variadic marker (last)
+      | '...'      # Variadic marker
 
     ABITY := BASETY | :IDENT
 
@@ -778,8 +778,8 @@ integer.  If the called function does not expect an environment
 parameter, it will be safely discarded.  See the <@ Functions >
 section for more information about environment parameters.
 
-When the called function is variadic, the last argument
-must be `...`.
+When the called function is variadic, there must be a `...`
+marker separating the named and variadic arguments.
 
 ~ Variadic
 ~~~~~~~~~~