libc/glibc: repair debugging with gdb (glibc 2.9)

Without this patch, crosstool-ng-built glibc-2.9 prevents
debugging any exeutable with gdb.
gdb says:
[Thread debugging using libthread_db enabled]
find_new_threads_callback: cannot get thread info: generic error
See also https://bugzilla.redhat.com/show_bug.cgi?id=487212
for a discussion of the bug and the solution.

# Compute Alpha-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple:

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

}