runtime: use monotonic clock for openbsd/386 and openbsd/amd64 timers

Switch nanotime to a monotonic clock on openbsd/386 and openbsd/amd64.
Also use a monotonic clock when for thrsleep, since the sleep duration
is based on the value returned from nanotime.

Update #6007

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/68460044

src/pkg/runtime/os_openbsd.c

@@ -82,7 +82,7 @@ runtime·semasleep(int64 ns)
 				// NOTE: tv_nsec is int64 on amd64, so this assumes a little-endian system.
 				ts.tv_nsec = 0;
 				ts.tv_sec = runtime·timediv(ns, 1000000000, (int32*)&ts.tv_nsec);
-				runtime·thrsleep(&m->waitsemacount, CLOCK_REALTIME, &ts, &m->waitsemalock, nil);
+				runtime·thrsleep(&m->waitsemacount, CLOCK_MONOTONIC, &ts, &m->waitsemalock, nil);
 			}
 			// reacquire lock
 			while(runtime·xchg(&m->waitsemalock, 1))

src/pkg/runtime/sys_openbsd_386.s

@@ -9,6 +9,8 @@
 #include "zasm_GOOS_GOARCH.h"
 #include "../../cmd/ld/textflag.h"
 
+#define	CLOCK_MONOTONIC	$3
+
 // Exit the entire program (like C exit)
 TEXT runtime·exit(SB),NOSPLIT,$-4
 	MOVL	$1, AX
@@ -133,7 +135,7 @@ TEXT time·now(SB), NOSPLIT, $32
 // void nanotime(int64 *nsec)
 TEXT runtime·nanotime(SB),NOSPLIT,$32
 	LEAL	12(SP), BX
-	MOVL	$0, 4(SP)		// arg 1 - clock_id
+	MOVL	CLOCK_MONOTONIC, 4(SP)	// arg 1 - clock_id
 	MOVL	BX, 8(SP)		// arg 2 - tp
 	MOVL	$87, AX			// sys_clock_gettime
 	INT	$0x80

src/pkg/runtime/sys_openbsd_amd64.s

@@ -9,6 +9,8 @@
 #include "zasm_GOOS_GOARCH.h"
 #include "../../cmd/ld/textflag.h"
 
+#define CLOCK_MONOTONIC	$3
+
 // int64 tfork(void *param, uintptr psize, M *mp, G *gp, void (*fn)(void));
 TEXT runtime·tfork(SB),NOSPLIT,$32
 
@@ -166,7 +168,7 @@ TEXT time·now(SB), NOSPLIT, $32
 	RET
 
 TEXT runtime·nanotime(SB),NOSPLIT,$24
-	MOVQ	$0, DI			// arg 1 - clock_id
+	MOVQ	CLOCK_MONOTONIC, DI	// arg 1 - clock_id
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$87, AX			// sys_clock_gettime
 	SYSCALL