libc/uClibc: fix snapshots

Snapshots are in a subdir named uClibc, not uClibc-snapshot
(or uClibc-YYYYMMDD either).
(transplanted from b13d0db6fa95795bb663bf674372b58221682dd6)

 # Compute IA-64-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}"

 }