Testsuite runs LLVM tests even if llc isn't present

Currently configure searches for llc but sets SettingsLlcCommand to llc if it isn't found. Then both make and Hadrian simply check whether the llc command is non-null to determine whether LLVM tests should be run. As a result, LLVM tests are always run, even if LLVM isn't available.

Originally noticed in !3767 (comment 291834).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information