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

When configure fails to find llc/opt within supported version range, it used to set "llc"/"opt" as fallback values. This behavior is particularly troublesome when the user has llc/opt 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 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.