cppia JIT: bring up the arm64 code generator #271

The cppia JIT is off on arm64 for 6.0.0. Until now its arm64 code had never run in CI: on Apple Silicon it could not get executable memory (#266), and Linux arm64 only runs with the Haxe nightly. With MAP_JIT in place it ran on macOS arm64 for the first time (run https://github.com/openfl/libhxcpp/actions/runs/39885412835). libhxcpp's own cppia suite passed under it, but Haxe's unit tests under cppia, which pass interpreted (10,982 assertions), ended as soon as they started:

src/unit/TestMain.hx:44: START
libc++abi: terminating due to uncaught exception of type Dynamic
Abort trap: 6           bin/cppia/Host-debug bin/unit.cppia -jit

Jitted code cannot unwind C++ exceptions, so every native helper it calls catches them (TRY_NATIVE and CATCH_NATIVE) and leaves them for the jitted code's checkException. The same tests pass under the JIT on x86 and x86-64, so either a helper only arm64 code reaches lets an exception through, or the arm64 code generator gets the exception path wrong.

To work on it, build with -D HXCPP_CPPIA_JIT_ARM64, which turns the JIT on for arm64. Next steps: run Haxe's unit tests under cppia with the JIT on macOS arm64 under lldb (.github/workflows/traced.sh), so the abort shows the native frame the exception came from and the jitted frame it hit; fix that; then run the cppia suite, the cppia regression tests and Haxe's unit tests under the JIT on macOS arm64 before turning it on by default.