ci: Add the targets/ subdirectory to facilitate cross-platform testing

Considering that a non-trivial amount of libpng code is arch-specific,
we should perform cross-platform builds (with cross-platform toolchains)
and test runs (on emulated architectures) in our routine verification.

The content of ci/targets/ shall consist of target description files,
written in the standard shell language. These files may be source'd as
needed, before running the verification scripts ci/ci_verify_*.sh.

Here is the initial list of target systems:
Android, Cygwin, FreeBSD, Linux, MSDOS, Windows.

And here is the initial list of target architectures:
ARM, MIPS, PowerPC, RISC-V, x86.

ci/targets/android/ci_env.i686-linux-android.sh

@@ -0,0 +1,16 @@
+# Copyright (c) 2023-2024 Cosmin Truta.
+#
+# Use, modification and distribution are subject to the MIT License.
+# Please see the accompanying file LICENSE_MIT.txt
+#
+# SPDX-License-Identifier: MIT
+
+export CI_TARGET_ARCH=i686
+export CI_TARGET_ARCHVER=i686
+export CI_TARGET_SYSTEM=linux
+export CI_TARGET_ABI=android
+export CI_TARGET_ABIVER=android29
+
+export CI_CC="$CI_TARGET_ARCHVER-$CI_TARGET_SYSTEM-$CI_TARGET_ABIVER-clang"
+export CI_AR="llvm-ar"
+export CI_RANLIB="llvm-ranlib"