libc/glibc: use CT_ARCH, in place of CT_KERNEL_ARCH

Since we merged the 32/64-bit archs, and we explicitly used
the Linux kernel arch name, we can get rid of CT_KERNEL_ARCH.

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

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

 }