vax __lwp_getprivate_fast: Fix asm constraints.

r0 is not clobbered; it is the output.  So say so.

No volatile is needed here, and volatile would be wrong, because
calls to __lwp_getprivate_fast can be safely deleted if the result is
not used.

PR port-vax/60101: vax: __lwp_getprivate_fast() inline asm uses
GCC-specific register variable pattern, not portable

Author: riastradh

sys/arch/vax/include/lwp_private.h

@@ -1,4 +1,4 @@
-/*	$NetBSD: lwp_private.h,v 1.1 2024/11/30 01:04:15 christos Exp $	*/
+/*	$NetBSD: lwp_private.h,v 1.2 2026/03/19 04:38:07 riastradh Exp $	*/
 
 /*-
  * Copyright (c) 2001 The NetBSD Foundation, Inc.
@@ -42,7 +42,7 @@ static __inline void *
 __lwp_getprivate_fast(void)
 {
 	register void *tcb __asm("r0");
-	__asm("chmk %0" :: "i"(SYS__lwp_getprivate) : "r0");
+	__asm("chmk %1" : "=r"(tcb) : "i"(SYS__lwp_getprivate));
 	return tcb;
 }