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>

src/runtime/cgo/gcc_freebsd_sigaction.c

@@ -29,7 +29,7 @@ x_cgo_sigaction(intptr_t signum, const go_sigaction_t *goact, go_sigaction_t *ol
 	int32_t ret;
 	struct sigaction act;
 	struct sigaction oldact;
-	int i;
+	size_t i;
 
 	_cgo_tsan_acquire();
 

src/runtime/cgo/gcc_sigaction.c

@@ -33,7 +33,7 @@ x_cgo_sigaction(intptr_t signum, const go_sigaction_t *goact, go_sigaction_t *ol
 	int32_t ret;
 	struct sigaction act;
 	struct sigaction oldact;
-	int i;
+	size_t i;
 
 	_cgo_tsan_acquire();