libroot/musl: adopt default implementation for a_crash from upstream

Change-Id: I30a1075163c1702755c86d0bd85beeb75ad894d2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6907
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>

src/system/libroot/posix/musl/internal/atomic.h

@@ -5,6 +5,14 @@
 
 #include "atomic_arch.h"
 
+#ifndef a_crash
+#define a_crash a_crash
+static inline void a_crash()
+{
+	*(volatile char *)0=0;
+}
+#endif
+
 #ifndef a_ctz_32
 #define a_ctz_32 a_ctz_32
 static inline int a_ctz_32(uint32_t x)