os: replace itod on posix with general itoa and fix possible infinite recursion

Remove use of itod on posix systems and replace with call to itoa.
Build and use same itoa function on all systems.
Fix infinite recursion in iota function for the case -1<<63.

Change-Id: I89d7e742383c5c4aeef8780501c78a3e1af87a6f
Reviewed-on: https://go-review.googlesource.com/2213


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>