[dev.cc] runtime: change set_sec to take int64

Fixes build.
Tested that all these systems can make.bash.

TBR=austin
CC=golang-codereviews
https://golang.org/cl/177770043
diff --git a/src/runtime/defs_linux_arm.go b/src/runtime/defs_linux_arm.go
index 7f83002..a874b15 100644
--- a/src/runtime/defs_linux_arm.go
+++ b/src/runtime/defs_linux_arm.go
@@ -84,8 +84,8 @@
 	tv_nsec int32
 }
 
-func (ts *timespec) set_sec(x int32) {
-	ts.tv_sec = x
+func (ts *timespec) set_sec(x int64) {
+	ts.tv_sec = int32(x)
 }
 
 func (ts *timespec) set_nsec(x int32) {