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.
37 files changed
