cgo: add support for C.float and C.double

They are not necessary in TinyGo because they always map to float32 and
float64, but it's a good idea to add them regardless for compatibility
with existing software.

(Now I think about it, perhaps it would have been better to require
explicit casts here just in case we want to support some really weird C
system, but then again we even force 64-bit double on AVR even though
avr-gcc defaults to 32-bit double).

cgo/testdata/types.go

@@ -112,6 +112,10 @@ import "C"
 import "C"
 
 var (
+	// aliases
+	_ C.float
+	_ C.double
+
 	// Simple typedefs.
 	_ C.myint