SECURITY: disable build of filter_neon.S on arm

This fixes the bug https://github.com/pnggroup/libpng/issues/505
"libpng does not support PAC/BTI on aarch64 targets" which arises
because the build mechanisms (both cmake and configure) assemble
arm/filter_neon.S even though it ends up completely empty.  The empty
file effectively poisons the so that the PAC/BTI support gets disabled.

The fix is minimal; it simply removes arm/filter_neon.S from the list of
sources included in the 64-bit ARM builds build.  Note that this was
already done in cmake for MSVC - it's not clear whether this change was
a partial fix for the same issue.

This version of the fix ONLY affects aarch64 (arm64) builds; 32-bit ARM
systems can still invoke the assembler if required and, indeed, there
should be no change whatsover to those builds.

The assembler code could not be used on 64-bit systems in any case so
in practice there is no material change to 64-bit builds either.

TESTING: pull the changes then type "autoreconf" if using configure (not
required for cmake).

TESTS: cmake has not been tested because cross-builds with cmake
currently fail to find the zlib installation from the cmake system root
path.  The following has been tested with configure cross builds:

armv7-linux-gnueabi [no neon support]
armv7a-linux-gnueabi [no neon support]
armv7a-hardfloat-linux-gnueabi [neon support not enabled]
armv7a-hardfloat-linux-gnueabi -mfpu=neon [uses intrinics]
armv7a-hardfloat-linux-gnueabi -mfpu=neon
        -DPNG_ARM_NEON_IMPLEMENTATION=2 [uses assembler]
aarch64-linux-gnu [uses intrinsics]
aarch64-linux-gnu -DPNG_ARM_NEON_OPT=0 [neon support disabled]

Signed-off-by: John Bowler <jbowler@acm.org>

configure.ac

@@ -425,6 +425,16 @@ AM_CONDITIONAL([PNG_ARM_NEON],
       *)    test "$enable_arm_neon" != '' ;;
     esac])
 
+# Add the assembler implementation source file.  This only works on 32-bit
+# ARM and causes problems even if empty on 64-bit ARM.
+AM_CONDITIONAL([PNG_ARM_NEON_ASM],
+   [test "$enable_arm_neon" != 'no' &&
+    case "$host_cpu" in
+      arm64*|aarch64*) false ;;
+      arm*) true ;;
+      *)    test "$enable_arm_neon" != '' ;;
+    esac])
+
 # MIPS MSA
 # ========