Add a calling convention to all function signatures.

A CallConv enum on every function signature makes it possible to
generate calls to functions with different calling conventions within
the same ISA / within a single function.

The calling conventions also serve as a way of customizing Cretonne's
behavior when embedded inside a VM. As an example, the SpiderWASM
calling convention is used to compile WebAssembly functions that run
inside the SpiderMonkey virtual machine.

All function signatures must have a calling convention at the end, so
this changes the textual IL syntax.

Before:

    sig1 = signature(i32, f64) -> f64

After

    sig1 = (i32, f64) -> f64 native
    sig2 = (i32) spiderwasm

When printing functions, the signature goes after the return types:

    function %r1() -> i32, f32 spiderwasm {
    ebb1:
        ...
    }

In the parser, this calling convention is optional and defaults to
"native". This is mostly to avoid updating all the existing test cases
under filetests/. When printing a function, the calling convention is
always included, including for "native" functions.

filetests/regalloc/spill.cton

@@ -93,7 +93,7 @@ ebb0(v0: i32):
 
 ; The same value used as indirect callee and argument.
 function %doubleuse_icall1(i32) {
-    sig0 = signature(i32)
+    sig0 = (i32) native
 ebb0(v0: i32):
     ; not:copy
     call_indirect sig0, v0(v0)
@@ -102,7 +102,7 @@ ebb0(v0: i32):
 
 ; The same value used as indirect callee and two arguments.
 function %doubleuse_icall2(i32) {
-    sig0 = signature(i32, i32)
+    sig0 = (i32, i32) native
 ebb0(v0: i32):
     ; check: $(c=$V) = copy $v0
     call_indirect sig0, v0(v0, v0)