configure: Search for LLVM executables with two-number versions
configure: Search for LLVM executables with two-number versions

Fedora uses the naming llc-7.0 while Debian uses llc-7. Ensure that both
are found.

Fixes #16990.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc