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/isa/intel/abi64.cton

@@ -6,14 +6,14 @@ isa intel
 ; regex: V=v\d+
 
 function %f() {
-    sig0 = signature(i32) -> i32
-    ; check: sig0 = signature(i32 [%rdi]) -> i32 [%rax]
+    sig0 = (i32) -> i32 native
+    ; check: sig0 = (i32 [%rdi]) -> i32 [%rax] native
 
-    sig1 = signature(i64) -> b1
-    ; check: sig1 = signature(i64 [%rdi]) -> b1 [%rax]
+    sig1 = (i64) -> b1 native
+    ; check: sig1 = (i64 [%rdi]) -> b1 [%rax] native
 
-    sig2 = signature(f32, i64) -> f64
-    ; check: sig2 = signature(f32 [%xmm0], i64 [%rdi]) -> f64 [%xmm0]
+    sig2 = (f32, i64) -> f64 native
+    ; check: sig2 = (f32 [%xmm0], i64 [%rdi]) -> f64 [%xmm0] native
 
 ebb0:
     return

filetests/isa/intel/binary32.cton

@@ -9,7 +9,7 @@ isa intel haswell
 
 function %I32() {
     fn0 = function %foo()
-    sig0 = signature()
+    sig0 = ()
 
 ebb0:
     ; asm: movl $1, %ecx

filetests/isa/intel/binary64.cton

@@ -11,7 +11,7 @@ isa intel haswell
 ; Tests for i64 instructions.
 function %I64() {
     fn0 = function %foo()
-    sig0 = signature()
+    sig0 = ()
 
 ebb0:
 
@@ -457,7 +457,7 @@ ebb2:
 ; be done by an instruction shrinking pass.
 function %I32() {
     fn0 = function %foo()
-    sig0 = signature()
+    sig0 = ()
 
 ebb0: