Legalize libcall signatures.

Explicitly legalize signatures created for libcalls.

cranelift/filetests/isa/x86/legalize-libcall.cton

@@ -3,6 +3,7 @@ test legalizer
 ; Pre-SSE 4.1, we need to use runtime library calls for floating point rounding operations.
 set is_64bit
 set is_pic
+set call_conv=system_v
 isa x86
 
 function %floor(f32) -> f32 {
@@ -11,6 +12,6 @@ ebb0(v0: f32):
     return v1
 }
 ; check: function %floor(f32 [%xmm0]) -> f32 [%xmm0] fast {
-; check: sig0 = (f32) -> f32 fast
+; check: sig0 = (f32 [%xmm0]) -> f32 [%xmm0] system_v
 ; check: fn0 = %FloorF32 sig0
 ; check: v1 = call fn0(v0)