abc is failing to compile on ppc64le because char is unsigned by
default:

src/misc/extra/extraUtilMisc.c: In function ‘void abc::Extra_TruthExpand(int, int, unsigned int*, unsigned int, unsigned int*)’:
src/misc/extra/extraUtilMisc.c:1550:5: error: narrowing conversion of ‘-1’ from ‘int’ to ‘char’ inside { } [-Wnarrowing]

Build CMake on GitHub Actions

Also, resolve CMake build problems on macOS:
Pass CMAKE_OSX_SYSROOT as an environment variable SDKROOT when buildind the arch_flags executable.

Move CI to GitHub Actions.

Also, a few minor changes that are required to compile ABC under moder compilers.

Consider unconnected carry-out ports

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>