configure: do not set LLC/OPT/LLVMAS fallback values when FIND_LLVM_PROG fails

When configure fails to find LLC/OPT/LLVMAS within supported version
range, it used to set "llc"/"opt"/"clang" as fallback values. This
behavior is particularly troublesome when the user has llc/opt/clang
with other versions in their PATH and run the testsuite, since hadrian
will incorrectly assume have_llvm=True and pass that to the testsuite
driver, resulting in annoying optllvm test failures (#23186). If
configure determines llc/opt/clang wouldn't work, then we shouldn't
pretend it'll work at all, and the bindist configure will invoke
FIND_LLVM_PROG check again at install time anyway.