[ClangBuilder] Pass -k to make for test-suite runs (#251)

When
https://github.com/llvm/llvm-lnt/commit/768874171529d31d12adb032df57b1c5f1c1fa77
enabled using Ninja, it removed the -k flag from make. -k causes make to
carry on if there's a build failure.

Without this flag, a single compilation failure turns into thousands of
missing executables in the final results. As subsequent targets are not
built.

https://github.com/llvm/llvm-lnt/pull/25 is adding an option to the lnt
runner to pass options to the build tool.

This PR uses that to pass -k to make to restore the old behaviour on the
build bots.