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.

cranelift/filetests/isa/riscv/legalize-abi.cton

@@ -106,7 +106,7 @@ ebb0(v0: i64x4):
 }
 
 function %indirect(i32) {
-    sig1 = signature()
+    sig1 = () native
 ebb0(v0: i32):
     call_indirect sig1, v0()
     return
@@ -114,7 +114,7 @@ ebb0(v0: i32):
 
 ; The first argument to call_indirect doesn't get altered.
 function %indirect_arg(i32, f32x2) {
-    sig1 = signature(f32x2)
+    sig1 = (f32x2) native
 ebb0(v0: i32, v1: f32x2):
     call_indirect sig1, v0(v1)
     ; check: call_indirect $sig1, $v0($V, $V)