arm: Add a placeholder file in lieu of the former filter_neon.S

In the previous commit 9e53875
we removed the obsolete assembler implementation `filter_neon.S`.

In this commit we add a stand-in for the original file, restoring
the original source tree structure, for the benefit of continuing
hassle-free libpng source upgrades in the 1.6.x line.

arm/filter_neon.S

@@ -0,0 +1,61 @@
+
+/* filter_neon.S - placeholder file
+ *
+ * Copyright (c) 2024 Cosmin Truta
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ */
+
+/* IMPORTANT NOTE:
+ *
+ * Historically, the hand-coded assembler implementation of Neon optimizations
+ * in this module had not been in sync with the intrinsics-based implementation
+ * in filter_neon_intrinsics.c and palette_neon_intrinsics.c, at least since
+ * the introduction of riffled palette optimizations. Moreover, the assembler
+ * code used to work on 32-bit ARM only, and it caused problems, even if empty,
+ * on 64-bit ARM.
+ *
+ * All references to this module from our internal build scripts and projects
+ * have been removed.
+ *
+ * For the external projects that might still expect this module to be present,
+ * we leave this stub in place, for the remaining lifetime of libpng-1.6.x.
+ * Everything should continue to function normally, as long as there are no
+ * deliberate attempts to use the old hand-made assembler code. A build error
+ * will be raised otherwise.
+ */
+
+/* This is required to get the symbol renames, which are #defines, and the
+ * definitions (or not) of PNG_ARM_NEON_OPT and PNG_ARM_NEON_IMPLEMENTATION.
+ */
+#define PNG_VERSION_INFO_ONLY
+#include "../pngpriv.h"
+
+#ifdef PNG_READ_SUPPORTED
+#if PNG_ARM_NEON_IMPLEMENTATION == 2 /* hand-coded assembler */
+#if PNG_ARM_NEON_OPT > 0
+
+#if defined(__clang__)
+#define GNUC_VERSION 0 /* not gcc, although it might pretend to be */
+#elif defined(__GNUC__)
+#define GNUC_MAJOR (__GNUC__ + 0)
+#define GNUC_MINOR (__GNUC_MINOR__ + 0)
+#define GNUC_PATCHLEVEL (__GNUC_PATCHLEVEL__ + 0)
+#define GNUC_VERSION (GNUC_MAJOR * 10000 + GNUC_MINOR * 100 + GNUC_PATCHLEVEL)
+#else
+#define GNUC_VERSION 0 /* not gcc */
+#endif
+
+#if (GNUC_VERSION > 0) && (GNUC_VERSION < 40300)
+#error "PNG_ARM_NEON is not supported with gcc versions earlier than 4.3.0"
+#elif GNUC_VERSION == 40504
+#error "PNG_ARM_NEON is not supported with gcc version 4.5.4"
+#else
+#error "Please use 'arm/*_neon_intrinsics.c' for PNG_ARM_NEON support"
+#endif
+
+#endif /* PNG_ARM_NEON_OPT > 0 */
+#endif /* PNG_ARM_NEON_IMPLEMENTATION == 2 */
+#endif /* READ */