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}"

 }