runtime: fix freebsd build

TBR=golang-dev
CC=golang-dev
https://golang.org/cl/13720044

src/pkg/runtime/sys_freebsd_amd64.s

@@ -28,7 +28,7 @@
 // but it expects the third argument in R10. Instead of rewriting
 // all the assembly in this file, #define SYSCALL to a safe simulation
 // using INT $0x80.
-/
+//
 // INT $0x80 is a little slower than SYSCALL, but correctness wins.
 //
 // See golang.org/issue/6372.