freebsd: Add stubs for runtime·osyield and runtime·usleep.

These don't do anything useful; they are just here to fix the build.

R=golang-dev
TBR=r
CC=golang-dev
https://golang.org/cl/5154048

src/pkg/runtime/freebsd/386/sys.s

@@ -174,6 +174,10 @@ TEXT runtime·sigaltstack(SB),7,$0
 	CALL	runtime·notok(SB)
 	RET
 
+// TODO: Implement usleep
+TEXT runtime·usleep(SB),7,$0
+	RET
+
 /*
 descriptor entry format for system call
 is the native machine format, ugly as it is:

src/pkg/runtime/freebsd/amd64/sys.s

@@ -169,6 +169,10 @@ TEXT runtime·sigaltstack(SB),7,$-8
 	CALL	runtime·notok(SB)
 	RET
 
+// TODO: Implement usleep
+TEXT runtime·usleep(SB),7,$0
+	RET
+
 // set tls base to DI
 TEXT runtime·settls(SB),7,$8
 	ADDQ	$16, DI	// adjust for ELF: wants to use -16(FS) and -8(FS) for g and m

src/pkg/runtime/freebsd/thread.c

@@ -199,3 +199,9 @@ runtime·sigpanic(void)
 	}
 	runtime·panicstring(runtime·sigtab[g->sig].name);
 }
+
+// TODO: fill this in properly.
+void
+runtime·osyield(void)
+{
+}