runtime: fix freebsd crash

FreeBSD, alone among our supported operating systems,
required that usleep not be interrupted.  Don't require that.

Fixes #3217.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5781045

src/pkg/runtime/sys_freebsd_386.s

@@ -220,8 +220,6 @@ TEXT runtime·usleep(SB),7,$20
 	MOVL	$0, 8(SP)		// arg 2 - rmtp
 	MOVL	$240, AX		// sys_nanosleep
 	INT	$0x80
-	JAE	2(PC)
-	CALL	runtime·notok(SB)
 	RET
 
 /*

src/pkg/runtime/sys_freebsd_amd64.s

@@ -205,8 +205,6 @@ TEXT runtime·usleep(SB),7,$16
 	MOVQ	$0, SI			// arg 2 - rmtp
 	MOVL	$240, AX		// sys_nanosleep
 	SYSCALL
-	JCC	2(PC)
-	CALL	runtime·notok(SB)
 	RET
 
 // set tls base to DI