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/parser/rewrite.cton

@@ -15,7 +15,7 @@ ebb100(v20: i32):
     v9200 = f64const 0x4.0p0
     trap
 }
-; sameln: function %defs() {
+; sameln: function %defs() native {
 ; nextln: $ebb100($v20: i32):
 ; nextln:     $v1000 = iconst.i32x8 5
 ; nextln:     $v9200 = f64const 0x1.0000000000000p2
@@ -29,7 +29,7 @@ ebb100(v20: i32):
     v200 = iadd v20, v1000
     jump ebb100(v1000)
 }
-; sameln: function %use_value() {
+; sameln: function %use_value() native {
 ; nextln: ebb0($v20: i32):
 ; nextln:     $v1000 = iadd_imm $v20, 5
 ; nextln:     $v200 = iadd $v20, $v1000