LLVM_HOST_TRIPLE is the triplet that llvm will be run on (build
is the build machine, host is the execution machine, target is
what it compiles for). This was not being set (only target was)
so it would default to the build machine.
This would result in issues with e.g. llvmpipe in mesa, which
would try to use the host triple to generate for, and subsequently
fail.
While at it, make sure not to clutter builddir when lldb is
unavailable.
Fixes https://github.com/void-linux/void-packages/issues/13896
56e5ac1249