Add a new manylinux_2_28-aarch64 cross-compilation image targeting
AlmaLinux 8 (glibc 2.28) for building portable Python wheels for
aarch64 from x86_64 hosts. Based on the manylinux_2_34-aarch64
variant with adjusted glibc (2.28) and kernel headers (4.18).
Also fix CMAKE_CROSSCOMPILING_EMULATOR in the 2_34 Toolchain.cmake
to use /usr/bin/qemu-aarch64-static (matching what the Dockerfile
actually installs).
Julian Oes