Improve cross-compilation for glibc aarch64 builds

[webmaster128]

As mentioned in https://github.com/CosmWasm/wasmvm/issues/293#issuecomment-1498815306, depending on old third party libc builds for the cross-compilation is not great. Since there does not seem to be sufficient cross-compilation tooling on CentOS, I'd like to change the build distro for the glibc aarch64 builds.

• [x] Split build_linux.sh in build_gnu_x86_64.sh and build_gnu_aarch64.sh (#438)
• [ ] Create an alternative Debian 11 based linux builder image Dockerfile.debian. This is a copy of Dockerfile.centos7 but should not need custom libraries. (#439)
• [ ] Check if both native and and cross builds work
• [ ] Use x86_64 build from CentOS 7 and aarch64 build from Debian 11
• [ ] Remove cross-compilation setup from Dockerfile.centos7

This bumps the min glibc version of system running it for the ARM build only. x86 users are unaffected.

[webmaster128]

As CentOS reached EOL, it is better to switch to Debian entirely for both aarch64 and x86_64. While much of the above is still valid, the architecture differentiation is not needed. Closing in favour of https://github.com/CosmWasm/wasmvm/issues/293.