libc/glibc: get the version string from version.h

We can not rely on the user-provided version string (be it via the
choice, or manually entered), so fallback to reading version.h,
which is both reliable and always present.

# Compute IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}