runtime: fix openbsd build

notetsleep: nosplit stack overflow
        120	assumed on entry to notetsleep
        96	after notetsleep uses 24
        88	on entry to runtime.semasleep
        32	after runtime.semasleep uses 56
        24	on entry to runtime.nanotime
        -8	after runtime.nanotime uses 32

Nanotime seems to be using only 24 bytes of stack space.
Unless I am missing something.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12041044

src/pkg/runtime/sys_openbsd_amd64.s

@@ -164,7 +164,7 @@ TEXT time·now(SB), 7, $32
 	MOVL	DX, nsec+8(FP)
 	RET
 
-TEXT runtime·nanotime(SB),7,$32
+TEXT runtime·nanotime(SB),7,$24
 	MOVQ	$0, DI			// arg 1 - clock_id
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$232, AX		// sys_clock_gettime