Expand unknown instructions to runtime library calls.

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

@@ -0,0 +1,15 @@
+test legalizer
+
+; Pre-SSE 4.1, we need to use runtime library calls for floating point rounding operations.
+set is_64bit
+isa intel
+
+function %floor(f32) -> f32 {
+ebb0(v0: f32):
+    v1 = floor v0
+    return v1
+}
+; check: function %floor(f32 [%xmm0]) -> f32 [%xmm0] native {
+; check: sig0 = (f32) -> f32 native
+; check: fn0 = sig0 %FloorF32
+; check: $v1 = call fn0($v0)