operand: fix integer float data (#393)

Issue #387 pointed out that integer float data is printed incorrectly, such
that it is not parsed correctly by the Go assembler. Specifically, integer
values need the decimal point, otherwise they will be treated as integers. For
example, 1 must be represented as `$(1.)` or `$(1.0)` to be parsed correctly.

This PR fixes that problem and adds a regression test.  The root of the
problem was that the formatting verb `%#v` does not have the right behavior
for integers. We fix it by deferring to custom `String()` function for the
float operand types.

Fixes #387
Closes #388

tests/fixedbugs/issue387/stub.go

@@ -0,0 +1,9 @@
+// Code generated by command: go run asm.go -out issue387.s -stubs stub.go. DO NOT EDIT.
+
+package issue387
+
+// Float32 indexes into an array of single-precision integral floats.
+func Float32(i int) float32
+
+// Float64 indexes into an array of double-precision integral floats.
+func Float64(i int) float64