libc/uclibc: 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.

 reporter_name="Matthias KAEHLCKE"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-08/msg00055.html"

 reporter_comment=""