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 Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }