tinderbox: If the clang lookup fails fallback to the old default behavior.

This fixes errors from the MK_CLANG_BOOTSTRAP/MK_LLD_BOOTSTRAP lookups
to not force using XCC/XLD but to rather just build them as normal by
allowing their own bootstrap logic to work.

MFC after: 3 weeks
X-MFC-with: r335711 r335769
Sponsored by: Dell EMC