*: generalize "const trick"

While at it, change all "__asm__" to "asm"

Co-authored-by: canyie <31466456+canyie@users.noreply.github.com>
Signed-off-by: YU Jincheng <shana@zju.edu.cn>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/appletlib.c

@@ -247,8 +247,7 @@ void lbb_prepare(const char *applet
 		IF_FEATURE_INDIVIDUAL(, char **argv))
 {
 #ifdef bb_cached_errno_ptr
-	(*(int **)not_const_pp(&bb_errno)) = get_perrno();
-	barrier();
+	ASSIGN_CONST_PTR(bb_errno, get_perrno());
 #endif
 	applet_name = applet;