Implement __atomic_is_lock_free for powerpc. Needed because the

architecture doesn't implement 64-bit atomic operations.  This
implementation is pessimistic and only flags naturally aligned
operations up to and including 32-bit as lock free.

tested by cwen@
ok gkoehler@

lib/libcompiler_rt/Makefile

@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile,v 1.14 2018/12/25 09:20:49 claudio Exp $
+# $OpenBSD: Makefile,v 1.15 2020/04/04 22:11:36 kettenis Exp $
 
 .include <bsd.own.mk>
 
@@ -275,7 +275,8 @@ SRCS+=	comparetf2.c \
 .endif
 
 .if ${RTARCH} == "ppc"
-SRCS+=	divtc3.c \
+SRCS+=	atomic_lock_free.c \
+	divtc3.c \
 	fixtfdi.c \
 	fixunstfdi.c \
 	floatditf.c \

lib/libcompiler_rt/ppc/atomic_lock_free.c

@@ -0,0 +1,22 @@
+/* Public domain. */
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#pragma redefine_extname __atomic_is_lock_free_c __atomic_is_lock_free
+
+bool
+__atomic_is_lock_free_c(size_t size, void *ptr)
+{
+	switch (size) {
+	case 1:
+		return true;
+	case 2:
+		return (((uintptr_t)ptr & 1) == 0);
+	case 4:
+		return (((uintptr_t)ptr & 3) == 0);
+	}
+
+	return false;
+}