Looks like c2go and gcc disagree about the exact meaning of the
usual arithmetic conversions, in a way that broke 9l's archreloc.
Fix it.

It's very hard for me to see why the original C code did not say
what c2go interpreted it to say, but apparently it did not.
This is why Go has explicit numerical conversions.

Change-Id: I75bd73afd1fa4ce9a53c887e1bd7d1e26ff43ae4
Reviewed-on: https://go-review.googlesource.com/6405Reviewed-by: Russ Cox <rsc@golang.org>