runtime/cgo: use size_t in sizeof result comparison

When a variable of type int is compared with sizeof's return
value, gcc warns:

  comparison between signed and unsigned integer expressions

Change the type of a couple loop indices that looped over sizeof from
int to size_t to silence the warnings.

Fixes #25411

Change-Id: I2c7858f84237e77945651c7b1b6a75b97edcef65
Reviewed-on: https://go-review.googlesource.com/113335


Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>