Make the CBackend actually get included in llc by using USEDLIBS instead of

USEDLIB as the variable to which "CBackend" is appended. The surrounding
if clause is safe because currently the configure script ensures that the
CBackend target is always added to TARGETS_TO_BUILD. By using a non-hard
coded construct in the makefile, we gain uniformity and the ability to
change the default set of targets by only changing the configure script.

llvm-svn: 21474