runtime/cgo: fix stackguard on FreeBSD/amd64

A cursory reading of the cgo code suggests this
should be necessary, though I don't have access
to a FreeBSD machine for testing.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/3746047

src/pkg/runtime/cgo/freebsd_amd64.c

@@ -37,6 +37,12 @@ threadentry(void *v)
 
 	ts.g->stackbase = (uintptr)&ts;
 
+	/*
+	 * libcgo_sys_thread_start set stackguard to stack size;
+	 * change to actual guard pointer.
+	 */
+	ts.g->stackguard = (uintptr)&ts - ts.g->stackguard + 4096;
+
 	/*
 	 * Set specific keys.  On FreeBSD/ELF, the thread local storage
 	 * is just before %fs:0.  Our dynamic 6.out's reserve 16 bytes