Bump glibc to v2.28 in the linux cross-toolchain (aarch64)

Linux-aarch64 needs glibc 2.28: https://bugzilla.mozilla.org/show_bug.cgi?id=1973877

We are building in stretch, which has glibc 2.24 (x86 needs 2.18, so we're good there).

However, for aarch64 we might want something more recent (that might even allow to simplify gcc-cross).